Infrared and electronic spectra are indispensable for understanding the structural and energetic properties of charged molecules and clusters in the gas phase. However, the presence of isomers can potentially complicate the interpretation of spectra, even if the target molecules or clusters are mass-selected beforehand. Here, we describe an instrument for spectroscopically characterizing charged molecular clusters that have been selected according to both their isomeric form and their mass-to-charge ratio. Cluster ions generated by laser ablation of a solid sample are selected according to their collision cross sections with helium buffer gas using a drift tube ion mobility spectrometer and their mass-to-charge ratio using a quadrupole mass filter. The mobility- and mass-selected target ions are introduced into a cryogenically cooled, three-dimensional quadrupole ion trap where they are thermalized through inelastic collisions with an inert buffer gas (He or He/N2 mixture). Spectra of the molecular ions are obtained by tagging them with inert atoms or molecules (Ne and N2), which are dislodged following resonant excitation of an electronic transition, or by photodissociating the cluster itself following absorption of one or more photons. An electronic spectrum is generated by monitoring the charged photofragment yield as a function of wavelength. The capacity of the instrument is illustrated with the resonance-enhanced photodissociation action spectra of carbon clusters (Cn+) and polyacetylene cations (HC2nH+) that have been selected according to the mass-to-charge ratio and collision cross section with He buffer gas and of mass-selected Au2+ and Au2Ag+ clusters.

Infrared and electronic spectra provide important structural and energetic information on charged molecules and clusters. However, obtaining infrared and electronic spectra of molecular ions in the gas phase is a challenging enterprise due to low ion densities and the potential presence of different molecular species that absorb across the same spectral range. A successful strategy for confronting these obstacles involves mass selection of the target molecular ions followed by resonance-enhanced photodissociation (REPD) of the ions, or of the ions tagged by weakly bound atoms or molecules (typically He, Ne, Ar, or N2), and detection of the photofragments.1–23 The advantages are that mass selection reduces ambiguity regarding the identity of the absorbing species and that the photofragment ions can be detected with high efficiency, conferring exceptional sensitivity. The approach, which works particularly well for molecular ions cooled in a supersonic expansion or in an ion trap, has been used to obtain infrared and electronic spectra for a vast array of charged molecules using light from tunable benchtop lasers and optical parametric oscillators (OPOs) and free electron lasers. Other forms of action spectroscopy, including laser induced reaction20 and laser suppression of clustering,24 which also possess the advantages of mass-selectivity and high sensitivity, have also been deployed to probe infrared and electronic transitions of molecular ions.

Notwithstanding the advantage of mass-selection prior to spectroscopic interrogation, ambiguities remain when the target ions exist as two or more isomers with distinct infrared or electronic spectra. One method of distinguishing spectral features associated with different isomers is through hole-burning spectroscopy, whereby a fixed wavelength laser beam (IR, visible, or UV) is used to deplete one isomer’s population with a second laser beam tuned to probe the remaining isomer(s).8–15 Although powerful, this approach requires several tunable lasers and becomes complicated if more than two isomers are present. Isomers of charged molecules, including C3H3+ and protonated phenol, have also been distinguished by the manner in which they establish non-covalent bonds with tag atoms or molecules and the infrared spectra of the tagged species.25,26 An alternative isomer-selective strategy involves preselecting the target isomers according to their collision cross sections (CCSs) with an inert buffer gas using some form of an ion mobility spectrometer (IMS) before mass selection and spectroscopic interrogation. This IMS-MS approach has been implemented using a drift tube ion mobility spectrometer,27–29 a field asymmetric ion mobility spectrometer (FAIMS),30–33 a differential mobility spectrometer (DMS),34,35 or variants of traveling wave ion mobility spectrometers (TW IMSs).13,36Instruments have also been developed in which photoisomerization or photodetachment of selected isomers was measured by photoexciting the target molecular ion between two ion mobility stages.36,37 Other instruments have been developed in which carbon cluster anions39 or biomolecule anions40 were mobility-selected prior to photoelectron spectroscopy.

In this paper, we describe a new tandem ion mobility mass spectrometer combined with a cryogenically cooled ion trap (IMS-MS-cryo) that is suitable for spectroscopically interrogating charged clusters that are created through laser ablation of a solid sample, which may exist as several different isomers. The cluster ions are selected according to their drift mobility in He buffer gas and their mass-to-charge ratio before being trapped and exposed to tunable light in a cryogenically cooled ion trap, with detection of charged photofragments separated in a time-of-flight-mass spectrometer. The requirement for an isomer-selective spectroscopic approach for charged clusters generated through laser ablation is apparent from previous ion mobility studies. For example, Cn+ clusters formed by laser ablating graphite have been found to exist as linear isomers for n = 3–10, monocycles for n ≥ 7, bicycles for n ≥ 21, and fullerenes for n ≥ 30, with the coexistence of different isomers over extended size ranges.41 Corresponding studies have discovered coexisting isomers for many other charged clusters generated through laser ablation, including Cn,42Sin±,43,44Agn+,45Aun±,46,47AgmAun+,48NbmOn±,49 and FenOn+ and FenOn+1+ clusters.50 Obtaining isomer-selective infrared and electronic spectra of these clusters is desirable because of their potential applications in materials science and, in some cases, their possible presence in interstellar space.

The IMS-MS-cryo apparatus described in this paper is similar in concept to instruments developed by Rizzo and co-workers in which ions are preselected according to their collision cross sections and mass-to-charge ratio prior to being trapped and probed with tunable laser radiation in a cryogenic ion trap13,29–31,36 However, these instruments have been developed primarily for characterizing charged biomolecules generated with an electrospray ionization source rather than charged clusters generated through laser ablation of a solid sample. The IMS-MS-cryo instrument also has aspects common to the instrument developed by Campbell and Dunk for spectroscopically probing carbon clusters generated using laser ablation but does not feature an ion mobility stage.51 

In the remainder of this paper, we first provide an overview of the arrangement for forming, selecting, and spectroscopically probing charged clusters and continue by describing the IMS-MS-cryo instrument in detail. Next, we illustrate the instrument’s performance by presenting ion mobility data and electronic spectra for Cn+ clusters and polyacetylene cations (HC2nH+), showing that the clusters can be selected and probed according to their mass-to-charge ratio and their collision cross section. As further examples, we present electronic spectra of the Au2+ dimer and Au2Ag+ trimer, demonstrating the instrument’s capacity for spectroscopically interrogating metal cluster cations.

The strategy for forming, selecting, and spectroscopically probing cluster ions can be appreciated from the schematic illustration of the instrument in Fig. 1(a). Although this figure refers to carbon cluster cations, the strategy is applicable to many other charged clusters. Ions generated by pulsed laser ablation of a solid sample (region 1) are propelled through helium buffer gas by an electric field where they are separated spatially and temporally according to their collision cross sections with the buffer gas (region 2). More extended ions have larger collision cross sections and traverse the drift tube more slowly than faster, more compact ions.

FIG. 1.

(a) Schematic representation of the tandem IMS-MS-cryo instrument. Carbon cluster cations are used to illustrate the strategy, which is applicable to a broader range of charged clusters. Ions are formed through laser ablation of a sample (region 1), separated in a drift tube ion mobility spectrometer filled with He buffer gas (region 2), accumulated in a hexapole ion guide (region 3), mass-selected in a quadrupole mass filter (region 4), and introduced into a quadrupole ion trap where they are cooled and possibly tagged with N2 molecules (region 5), before being exposed to one or more pulses of tunable wavelength radiation. Ions are ejected from the trap and photofragments are separated according to their mass-to-charge ratio in a time-of-flight mass spectrometer (region 6). (b) Sectioned three-dimensional view of the IMS-MS-cryo instrument. The off-axis scintillator detector between the QMF and octupole ion guide is not shown. TMP = turbomolecular pump, B–N = Bradbury–Nielsen, QMF = quadrupole mass filter, and TOFMS = time-of-flight mass spectrometer.

FIG. 1.

(a) Schematic representation of the tandem IMS-MS-cryo instrument. Carbon cluster cations are used to illustrate the strategy, which is applicable to a broader range of charged clusters. Ions are formed through laser ablation of a sample (region 1), separated in a drift tube ion mobility spectrometer filled with He buffer gas (region 2), accumulated in a hexapole ion guide (region 3), mass-selected in a quadrupole mass filter (region 4), and introduced into a quadrupole ion trap where they are cooled and possibly tagged with N2 molecules (region 5), before being exposed to one or more pulses of tunable wavelength radiation. Ions are ejected from the trap and photofragments are separated according to their mass-to-charge ratio in a time-of-flight mass spectrometer (region 6). (b) Sectioned three-dimensional view of the IMS-MS-cryo instrument. The off-axis scintillator detector between the QMF and octupole ion guide is not shown. TMP = turbomolecular pump, B–N = Bradbury–Nielsen, QMF = quadrupole mass filter, and TOFMS = time-of-flight mass spectrometer.

Close modal

At the end of the ion mobility drift region, the ions encounter an electrostatic ion gate that can be opened to transmit a packet of ions with a defined range of velocities (i.e., mobilities or collision cross sections). Following the ion gate, the ions are collected radially by an RF ion funnel before passing through an orifice into a hexapole ion guide (region 3) and then through a quadrupole mass filter (region 4) where they can be mass selected. From here, the ion beam can be deflected to an off-axis scintillator detector, which is connected to a multichannel scaler (MCS) that can be used to generate a histogram of ion counts against arrival time, giving an arrival time distribution (ATD) for the ions.

Alternatively, the ion beam is allowed to continue through an octupole ion guide into a radio frequency quadrupole ion trap (QIT, region 5) that can be cooled to a temperature of 4 K. The ions are collected and confined in the QIT where they are thermalized through inelastic collisions with an inert buffer gas (either He, Ne, 1% N2 in He, or 1% N2 in H2). As they cool, weakly bound complexes form between the ions and buffer gas atoms or molecules. The clusters are overlapped with light from a tunable optical parametric oscillator (OPO), which, if tuned to an electronic transition, induces photodissociation. Following their irradiation, the ions are ejected from the QIT into a time-of-flight mass spectrometer (TOFMS, region 6) and are detected by a dual microchannel plate (MCP) detector. Normally, the machine is run with a duty cycle of 2 Hz with alternate ion packets exposed to the OPO beam. Subtracting the light-off signal from the light-on signal affords the overall photoresponse, which, when plotted against wavelength, gives the electronic action spectrum.

As described above, isomer separation is accomplished using a drift tube ion mobility spectrometer in which the ions are propelled by an electric field E through a buffer gas, attaining a velocity, vd. In the low field regime,

vd=KE,
(1)

where K, the mobility, is inversely proportional to the collision integral [Ω(1,1)(T)] and is given by52 

K=3ze16N2πμkBT121Ω(1,1)(T).
(2)

Here, z is the charge state of the ion, e is the elementary charge, kB is the Boltzmann constant, T is the absolute temperature, and N is the number density of the buffer gas molecules. The reduced mass μ is given by

μ=MmM+m,
(3)

where m is the mass of the ion and M is the mass of the buffer gas atoms. The collision integral [Ω(1,1)(T)], which is related to the deflection angle of the colliding partners, averaged over velocity, collision orientation, and impact parameter, depends on the intermolecular potential energy surface for the interaction between the cluster ion and buffer gas atom or molecule and is therefore sensitive to the cluster’s isomeric structure.52 For standardized reporting and comparison, the measured mobility is normally converted to the reduced mobility, K0,

K0=KP760273.2T,
(4)

where P is the pressure in Torr.

The construction of the IMS-MS-cryo instrument can be appreciated from Fig. 1(b). The first section of the machine is a laser ablation ion source connected to a drift tube ion mobility mass spectrometer and an ion funnel. This region is filled with He buffer gas (pressure ≈3 Torr) delivered at 0.2 SLM through a mass flow controller (Sevenstar, D08-1F) and pumped by a 12 m3/h rotary vane pump. Ions are produced by ablating the surface of a sample disk (ϕ = 20 mm) using the frequency doubled output (532 nm) of a pulsed Nd:YAG laser (Quantel Ultra) operating at a repetition rate of 20 or 50 Hz with a pulse energy of ≈12 mJ and a pulse duration of 5 ns. The laser beam is focused on the sample disk surface using a 500 mm focal length lens. The sample disk is mounted on a hypocycloidal stage whose 0.1 Hz rotation is driven by a geared 12 V DC motor. The path of the ablation laser spot on the disk is a precessing ellipse, ensuring that the material is removed evenly from the surface.

The laser-ablated cluster ions immediately enter the drift region where they are propelled by an electric field through He buffer gas. The first stage of the drift region is 10 cm long and consists of 12 evenly spaced ring electrodes (inner diameter 40 mm) that are fabricated from a printed circuit board (PCB) and are coupled by 1 MΩ resistors. An electric field of 11 V/cm is established by applying electrical potentials of 550 and 440 V to the first and the last electrodes, respectively. Following the ring electrode stack, the ions enter a 30 cm resistive glass tube with an inner diameter of 40 mm (Photonis),53 which has a resistance of 33 MΩ. The entrance and exit ends of the tube are biased at 440 and 170 V, respectively, generating a drift field of 9 V/cm. After exiting the glass drift tube, the ions encounter an electrostatic Bradbury–Nielsen ion gate consisting of interleaved tungsten wires54 that can be opened with an appropriate delay and width to transmit the desired isomer packet while blocking slower or faster isomers. Following the ion gate, the ions are gathered radially by an ion funnel (length = 10 cm). The ion funnel comprises 45 ring electrodes fabricated on PCB with electrode inner diameters that decrease linearly from the entrance (ϕ = 40 mm) to exit (ϕ = 7 mm). The electrodes are connected by 1 MΩ resistors with appropriate potentials applied to the first and last electrodes to establish a drift field of 8 V/cm. The electrodes are capacitively coupled to the RF supply (drive amplitude: 70 Vpp and frequency: 808 kHz).

After exiting the ion funnel, the ions pass through a 1 mm orifice in an electrode on a PCB, which serves as a differential wall between the drift region and hexapole region, and enter an RF hexapole ion guide. The hexapole drive voltage is 80 Vpp with a frequency of 800 kHz. By changing its DC electrical potential, the hexapole can be used either as a simple ion guide or as a trap to store ions from several ablation laser pulses. The former operation mode is used for measuring ATDs of the laser-ablated clusters, whereas the latter mode is used if the ions are to be transferred to the QIT for spectroscopic interrogation. The hexapole chamber is pumped by two 240 l/s turbomolecular pumps, which maintain a pressure of 5×105 Torr during operation.

The ions pass from the hexapole through a 3 mm aperture in a plate electrode into the QMF, where they are mass selected. The chamber containing the QMF is evacuated by a 240 l/s turbomolecular pump and has an operating pressure of 2×106 Torr. Following mass-selection in the QMF, the ions can be deflected to an off-axis scintillator detector that is connected to a multichannel scaler (MCS, FAST ComTec, MCS-4), which can be used to record an arrival time distribution (ATD) of the ions. The IMS stage, when operating with He buffer gas, possesses a mobility resolution of ≈20, which is sufficient to distinguish carbon cluster chains, rings, bi-rings, and fullerenes for clusters containing up to 70 carbon atoms. The resolution of the QMF can be adjusted to be ≤0.5 amu for clusters up to C60+, albeit with some sacrifice of transmission.

For spectroscopic characterization, the ions are transferred by an octupole ion guide (drive voltage: 28 Vpp and frequency: 800 kHz) through three electrostatic lenses to the QIT (R. M. Jordan TOF Products, Inc. C-1251). The arrangement for cooling the QIT follows the scheme described in Ref. 55. The QIT is driven at a frequency of 1 MHz with a typical amplitude of 500–900 Vpp. The original stainless steel electrodes of the QIT were replaced with oxygen-free copper electrodes to achieve better thermal conductivity and were sputter coated with gold (thickness ≈150 nm) to reduce accumulation of surface charge. The QIT is cooled using a closed-cycle cryo-cooler unit (RDK-205E 4K, Sumitomo Heavy Industries, Ltd.). The trapped ions are vibrationally and rotationally relaxed through inelastic collisions with a buffer gas (either He, Ne, 1% N2 in H2, or 1% N2 in He) delivered through a pulsed solenoid valve (General Valves, Series 9) mounted on the chamber wall. The outlet of the pulsed valve is connected by a Teflon tube to a section of a copper tube mounted on the copper cryotrap shield where the buffer gas is pre-cooled before passing through a thin Delrin tube inserted into a small hole in the QIT ring electrode. The pulsed valve is normally opened for ≈200 µs at the beginning of the trapping cycle and ≈20 ms before the ions are released from the hexapole ion guide. The chamber containing the QIT is evacuated by a 900 l/s turbomolecular pump and has an operational pressure of 106 Torr.

The QIT temperature can be adjusted by passing current through resistors attached to the QIT mount. The temperature is monitored using a Lakeshore DT-670 silicon diode attached to the mount. A QIT temperature of ≈25 K worked well for generating N2-tagged clusters using a gas mixture of either 1% N2 in He or 1% N2 in H2. Eventually, after ≈8 h, the QIT electrodes are normally coated with a layer of solid N2, affecting operation and necessitating heating of the trap to 80 K, before re-cooling and resuming operation.

For mass analysis and detection, the molecular ions are ejected from the QIT into the acceleration region of a 0.9 m linear time-of-flight mass spectrometer (TOFMS). The electrical potentials to the first and second acceleration grids of the TOFMS are supplied through two high-voltage switches (Behlke HTS 61-02) with the electrical potentials adjusted to achieve Wiley–McLaren space focusing. The ions are detected at the end of the flight tube by a tandem microchannel plate (MCP) assembly, the output of which is sent to a computer-based oscilloscope (Picoscope 5000). The TOFMS has a mass resolution of mm ≈ 1000, sufficient to distinguish neighboring mass peaks for carbon clusters containing different numbers of 13C atoms, at least up to C60+.

The ions are separated temporally and spatially according to their m/z over the short section between the QIT and the source region of the TOFMS—this section acts as a short TOFMS. The optimal delay between ejection of ions from the QIT and firing of the TOFMS acceleration grids depends on the ion mass and is adjusted to maximize the ion signal recorded at the MCP detector of the TOFMS. Several digital delay generators (Quantum Composers 9400 Series) are used to synchronize the triggering of the ablation laser, ejection of ions from the hexapole trap, opening of the gas nozzle, firing of the photodissocation laser pulses, ejection of ions from the QIT, and switching of the TOFMS acceleration potentials.

Resonance-enhanced photodissociation spectra are obtained by measuring the target cluster ion signal and photofragment ion signal as a function of the excitation wavelength. Usually, ions generated by laser ablation are stored in the hexapole and are released at a rate of 2 Hz through the QMF into the QIT. After 480 ms, the ions are ejected from the QIT into the TOFMS for mass analysis. In the QIT, alternate ion packets are exposed to unfocussed light from a tunable optical parametric oscillator (EKSPLA NT342B, 5 cm−1 bandwidth, 5 ns pulse width). Resonant photoexcitation causes destruction of the parent clusters and the corresponding formation of photofragment ions. Normally, the intensity of the OPO beam and the number of OPO shots delivered to the ion packet are adjusted to ensure that ≤5% of the ions are photodissociated when the light is tuned to the most intense transition. Typically, to accomplish this, the trapped ions are exposed to only a single pulse of light from the OPO that was selected using a mechanical shutter. However, for weaker transitions or in spectral regions where the OPO output is low, the ions are exposed to several light pulses. Under these conditions, the photofragment signal plotted against wavelength gives the REPD spectrum.

It should be emphasized that the REPD action spectrum represents a convolution of the cluster’s absorption spectrum and a function describing the wavelength-dependent photodissociation yield. The difference between the absorption spectrum and the REPD spectrum can be minimal if the target cluster is tagged by a weakly bound atom or molecule (e.g., He, Ne, Ar or N2) or if the photon energy exceeds the dissociation energy and there is a viable path to dissociation that is more efficient than competing deactivation processes. In contrast, there can be substantial differences between the REPD spectrum and the absorption spectrum if the accessed vibrational or electronic state lies below the dissociation threshold such that only clusters already possessing vibrational energy or which absorb several photons are able to photodissociate.

The performance of the IMS-MS section of the instrument can be exemplified with arrival time distributions for carbon cluster cations formed by laser ablation of a graphite disk. To do this, the machine is operated with the ions passing directly through the hexapole ion guide and being detected by the off-axis detector situated after the QMF [see Fig. 1(a)]. The isomeric distribution of Cn+ clusters with n = 10–40 is illustrated in Fig. 2, where the ion signal is plotted as a function of arrival time and mass-to-charge ratio. The data are consistent with previous ion mobility measurements from Ref. 41, albeit with slightly better resolution of the ATD peaks. Peaks associated with monocyclic carbon clusters are apparent for n ≥ 10, bicyclic clusters for n ≥ 21 and fullerenes for n ≥ 32. Importantly, for some size ranges, there is a coexistence of two or more isomers. For example, C36+ exists as monocyclic, bicyclic, and fullerene isomers (see Fig. 2). We also observe weak signals for hydrogenated carbon clusters, including the linear polyacetylene cations HC2nH+, which presumably arise from the reaction of carbon clusters formed in the laser ablation ion source with trace H2O.

FIG. 2.

Ion signal as a function of m/z and drift time for carbon cluster cations formed by laser ablation. Peaks are apparent for Cn+, CnH+, and CnH2+ ions.

FIG. 2.

Ion signal as a function of m/z and drift time for carbon cluster cations formed by laser ablation. Peaks are apparent for Cn+, CnH+, and CnH2+ ions.

Close modal

Table I summarizes measured reduced drift mobilities (K0) for monocyclic carbon rings (Cn+) and linear polyacetylene chains (HCnH+) over the n = 10–16 range, derived from the measured arrival times adjusted for the time the ions spend in the hexapole and QMF and calibrated with respect to the reported experimental value for C60+ (4.321 cm2 V−1 s−1; Ref. 56). To confirm assignment of the peaks to monocyclic carbon rings (Cn+) and polyacetylene chains (HCnH+), reduced drift mobilities (K0) for these structures were calculated using the IMoS 2.0 program suite57 with the trajectory method parameterized for He buffer gas at T = 298 K. Molecular geometries and input charge distributions were computed at the ωB97X-D/cc-pVTZ level of density functional theory, with the Merz–Singh–Kollman scheme parameterized to reproduce the electric dipole moments of the ions.58 The calculated reduced mobilities agree well with the experimental values (Table I), supporting assignments of the ATD peaks to monocyclic carbon rings (Cn+) and linear polyacetylene chains (HCnH+).

TABLE I.

Calculated and measured reduced mobilities (K0) for monocyclic C2n+ clusters and HC2nH+ (n = 5–8) clusters in He buffer gas (units cm2 V−1 s−1). Measured mobilities are calibrated with respect to the reported experimental value for C60+ (4.321 cm2 V−1 s−1; Ref. 56).

SpeciesK0 (calc.)K0 (meas.)meas./calc.
C10+ 9.80 10.00 1.02 
HC10H+ 8.04 8.20 1.02 
C12+ 8.44 8.55 1.01 
HC12H+ 6.96 7.13 1.02 
C14+ 7.34 7.46 1.01 
HC14H+ 6.17 6.27 1.02 
C16+ 6.47 6.59 1.02 
HC16H+ 5.52 5.62 1.02 
SpeciesK0 (calc.)K0 (meas.)meas./calc.
C10+ 9.80 10.00 1.02 
HC10H+ 8.04 8.20 1.02 
C12+ 8.44 8.55 1.01 
HC12H+ 6.96 7.13 1.02 
C14+ 7.34 7.46 1.01 
HC14H+ 6.17 6.27 1.02 
C16+ 6.47 6.59 1.02 
HC16H+ 5.52 5.62 1.02 

As a first example of the IMS-MS-cryo instrument’s capacity to spectroscopically characterize clusters with different masses and isomeric structures, we present data for the cyclic C14+ cluster and the linear HC14H+ polyacetylene cation. ATDs recorded with the QMF set to m/z 168 (corresponding to the major C14+ isotopologue) and m/z 170 (corresponding to the major HC14H+ isotopologue) are shown in Fig. 3, where it can be seen that the linear HCnH+ polyacetylene molecules are ≈0.35 ms slower than the more compact cyclic C14+ clusters. Note that a peak due to cyclic C14+ clusters containing two 13C atoms or to a cyclic version of HC14H+ is apparent in the ATD obtained with the QMF tuned to m/z 170. The clear mobility separation of the C14+ clusters and the linear HC14H+ polyacetylene molecules makes it straightforward to select either species using the Bradbury–Nielsen gate at the end of the ion mobility drift region.

FIG. 3.

ATDs for monocyclic C14+ (m/z 168) and linear HC14H+ (m/z 170) in He buffer gas. The C14+ peak in the m/z 170 ATD is presumably due to C14+ clusters containing two 13C atoms, leakage of C14+ clusters containing one 13C atom, and possibly cyclic C14H2+ clusters.

FIG. 3.

ATDs for monocyclic C14+ (m/z 168) and linear HC14H+ (m/z 170) in He buffer gas. The C14+ peak in the m/z 170 ATD is presumably due to C14+ clusters containing two 13C atoms, leakage of C14+ clusters containing one 13C atom, and possibly cyclic C14H2+ clusters.

Close modal

The REPD spectra of HC14H+ and of C14+ selectively introduced into the cryogenically cooled QIT and tagged with N2 are shown in Fig. 4. The HC14H+–N2 spectrum exhibits strong bands in the near infrared spectral range that correspond to bands previously reported for the linear polyacetylene HC14H+ cation trapped in a Ne matrix.58,59 In contrast, as previously reported,61 the main electronic absorptions of the monocyclic C14+–N2 cluster occur in the visible region (611.9 nm for the origin transition) with no discernible transitions in the infrared range.

FIG. 4.

REPD spectra of linear HC14H+–N2 and monocyclic C14+–N2. Red arrows indicate Ã2ΠgX̃2Πu absorptions of HC14H+ in a Ne matrix (1047.1 and 1019.5 nm from Ref. 59), while the green arrow indicates a D̃2ΠgX̃2Πu absorption of HC14H+ in a Ne matrix (609.8 nm from Ref. 60). Note the x-axis break.

FIG. 4.

REPD spectra of linear HC14H+–N2 and monocyclic C14+–N2. Red arrows indicate Ã2ΠgX̃2Πu absorptions of HC14H+ in a Ne matrix (1047.1 and 1019.5 nm from Ref. 59), while the green arrow indicates a D̃2ΠgX̃2Πu absorption of HC14H+ in a Ne matrix (609.8 nm from Ref. 60). Note the x-axis break.

Close modal

To demonstrate the IMS-MS-cryo instrument’s capacity for obtaining electronic spectra of isomer-selected cluster ions, we present data for C28+ clusters, which are known to have monocyclic and bicyclic isomers.40,60 The ATD recorded with the QMF set to m/z 336 shown in Fig. 5 exhibits peaks associated with monocyclic and bicyclic C28+ isomers and a small fraction of multicharged C562+ and C843+ fullerenes. The slight asymmetry of the bicyclic isomer ATD peak is presumably due to the presence of isomers with the carbon atoms distributed between the two rings in different ways. The second and third traces show ATDs recorded when either the monocyclic isomer or bicyclic isomer(s) were mobility-selected using the electrostatic Bradbury–Nielsen gate at the end of the drift region, demonstrating clean selection of either isomer.

FIG. 5.

(a) ATD for mass-selected C28+ clusters with the QMF set to m/z 336. (b) ATD for mobility-selected monocyclic C28+ clusters. (c) ATD for mobility-selected bicyclic C28+ clusters.

FIG. 5.

(a) ATD for mass-selected C28+ clusters with the QMF set to m/z 336. (b) ATD for mobility-selected monocyclic C28+ clusters. (c) ATD for mobility-selected bicyclic C28+ clusters.

Close modal

REPD electronic spectra measured over the 710–1400 nm wavelength range for C28+ ions contained in the QIT and tagged with N2 molecules are presented in Fig. 6. Figures 6(a) and 6(b) show electronic spectra of C28+ without isomer selection, obtained at low and high laser power, respectively. In the low-power spectrum, the peaks appear with their natural intensities, whereas weak spectral features are exaggerated in the high-power spectrum. Electronic spectra measured for mobility-selected C28+ monocyclic and bicyclic isomers are shown in Figs. 6(c) and 6(d), respectively. As reported previously,61 the monocyclic C28+ spectrum is dominated by an origin transition at 7576 cm−1 (±20 cm−1) and weaker transitions lying to shorter wavelength that are presumably due to in-plane ring deformation vibrational modes, with the peak at 9822 cm−1 (displaced from the origin by 2246 cm−1) tentatively assigned to a C≡C stretch vibrational mode. The monocyclic C28+ spectrum (obtained at low laser power) resembles the low-power spectrum measured with no gating [Fig. 6(a)], indicating that the bicyclic isomer, despite having a comparable abundance to the monocyclic isomer, does not absorb significantly over this range. Indeed, it was necessary to expose the bicyclic C28+ isomer(s) with much higher laser powers to record the spectrum shown in Fig. 6(d). Without mobility selection, the bicyclic C28+ isomer’s weak features would be totally obscured by the stronger monocyclic absorptions apparent in Figs. 6(a)6(c). It is only when the bicyclic form is selectively introduced into the QIT that it is possible to record the electronic spectrum shown in Fig. 6(d). The bicyclic isomer spectrum does not possess a clear origin band or easily assignable vibronic peaks, although it is possible that other transitions lie outside the investigated spectral range (710–1400 nm). Furthermore, it is possible that several bicyclic isomers with similar CCSs, which are difficult to separate using the IMS, contribute to the spectrum.

FIG. 6.

(a) Electronic spectrum for C28+ clusters that were not isomer-selected (low laser power). (b) Electronic spectrum for C28+ clusters that were not isomer-selected (high laser power). (c) Electronic spectrum for isomer-selected monocyclic C28+ clusters (low laser power). (d) Electronic spectrum for isomer-selected bicyclic C28+ clusters (high laser power). Spectra were recorded by exposing C28+–(N2)m complexes to tunable light in the QIT and monitoring C28+ photofragments.

FIG. 6.

(a) Electronic spectrum for C28+ clusters that were not isomer-selected (low laser power). (b) Electronic spectrum for C28+ clusters that were not isomer-selected (high laser power). (c) Electronic spectrum for isomer-selected monocyclic C28+ clusters (low laser power). (d) Electronic spectrum for isomer-selected bicyclic C28+ clusters (high laser power). Spectra were recorded by exposing C28+–(N2)m complexes to tunable light in the QIT and monitoring C28+ photofragments.

Close modal

To demonstrate the suitability of the instrument for spectroscopically probing metal clusters, we present the REPD spectra of Au2+ and Au2Ag+ over the 305–350 nm wavelength range (Fig. 7), where the target cation clusters were created by laser ablation of a gold/silver alloy disk. The spectra were obtained with pure He buffer gas in the cryogenically cooled QIT. The Au2+ spectrum, which was obtained by monitoring the Au+ photofragment yield as a function of wavelength, is similar to the spectrum reported in Ref. 62 and exhibits a complex pattern of peaks with no clear vibronic progressions. The Au2Ag+ spectrum, which was recorded on the AuAg+ photofragment channel, shows sharp spectral features across several band systems that correspond to broad, unresolved bands reported by Shayeghi and co-workers.63 Analysis of the Au2Ag+ spectrum is in progress. Although the isomer selectivity of the instrument is not required to study the metal dimers and trimers, larger gold and silver clusters, including Au4+,64 are predicted to possess several isomers with similar energy64,65 and should be suitable spectroscopic targets for the instrument.

FIG. 7.

(a) REPD electronic spectrum of Au2+ recorded by monitoring Au+ photofragments as a function of wavelength. (b) REPD electronic spectrum of Au2Ag+ recorded by monitoring AuAg+ photofragments as a function of wavelength.

FIG. 7.

(a) REPD electronic spectrum of Au2+ recorded by monitoring Au+ photofragments as a function of wavelength. (b) REPD electronic spectrum of Au2Ag+ recorded by monitoring AuAg+ photofragments as a function of wavelength.

Close modal

The IMS-MS-cryotrap instrument provides a means for measuring electronic spectra of cooled, isomer-selected, charged molecules and clusters in the gas phase. The machine’s capabilities have been demonstrated through several examples. First, it was shown that the cyclic C14+ carbon cluster and the linear HC14H+ polyacetylene cluster could be separated according to their masses and mobilities prior to spectroscopic interrogation in the cryogenic ion trap. Second, it was demonstrated that monocyclic and bicyclic isomers of C28+ could be cleanly separated and that the two isomers have distinct REPD spectra. Without isomer separation prior to spectroscopic interrogation in the QIT, it would be extremely difficult to obtain the relatively weak spectrum of the bicyclic C28+ isomer over the 700–1200 nm range. As a final example, we showed that it was possible to generate the Au2+ dimer and Au2Ag+ trimer and measure their electronic transitions, demonstrating that metal clusters can be formed and spectroscopically interrogated with the instrument.

In the future, the apparatus will be deployed for obtaining infrared and electronic spectra of diverse charged molecules and clusters composed of carbon, silicon, metal, and hydrogen atoms, where the ability to select species according to both mass-to-charge ratio and collision cross section should be useful for resolving ambiguities in assigning spectral features to particular isomers and for teasing out weak spectral features associated with minor isomers that may otherwise be obscured by stronger absorptions of more abundant isomers. It is also worth remarking that the laser ablation source should be suitable for producing a broad range of molecular ions through matrix assisted laser desorption/ionization (MALDI). Finally, we note that although the performance of the instrument has been illustrated with cation clusters, reconfiguring for operation with anion clusters is straightforward and merely involves reversing the DC potentials applied to all electrodes.

The authors thank Richard Mathys of the Science Faculty Workshop for exceptional contributions to the design and construction of the apparatus. This research was supported under the Australian Research Council’s Discovery Project funding scheme (Project Nos. DP150101427 and DP160100474). J.T.B. acknowledges the University of Melbourne and Australian government for support through the Research Training Program (RTP) scholarship scheme.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

1.
M.
Okumura
,
L. I.
Yeh
, and
Y. T.
Lee
,
J. Chem. Phys.
88
,
79
(
1988
).
2.
D. W.
Boo
and
Y. T.
Lee
,
Int. J. Mass Spectrom. Ion Processes
159
,
209
(
1996
).
3.
E. J.
Bieske
,
A.
Soliva
,
M. A.
Welker
, and
J. P.
Maier
,
J. Chem. Phys.
93
,
4477
(
1990
).
4.
M. A.
Duncan
,
Int. J. Mass Spectrom.
200
,
545
(
2000
).
5.
E. J.
Bieske
and
O.
Dopfer
,
Chem. Rev.
100
,
3963
(
2000
).
6.
A.
Dzhonson
,
D.
Gerlich
,
E. J.
Bieske
, and
J. P.
Maier
,
J. Mol. Struct.
795
,
93
(
2006
).
7.
J.
Jašík
,
J.
Žabka
,
J.
Roithová
, and
D.
Gerlich
,
Int. J. Mass Spectrom.
354–355
,
204
(
2013
).
8.
J. A.
Stearns
,
S.
Mercier
,
C.
Seaiby
,
M.
Guidi
,
O. V.
Boyarkin
, and
T. R.
Rizzo
,
J. Am. Chem. Soc.
129
,
11814
(
2007
).
9.
T. R.
Rizzo
,
J. A.
Stearns
, and
O. V.
Boyarkin
,
Int. Rev. Phys. Chem.
28
,
481
(
2009
).
10.
H.
Kang
,
G.
Féraud
,
C.
Dedonder-Lardeux
, and
C.
Jouvet
,
J. Phys. Chem. Lett.
5
,
2760
(
2014
).
11.
A. B.
Wolk
,
C. M.
Leavitt
,
E.
Garand
, and
M. A.
Johnson
,
Acc. Chem. Res.
47
,
202
(
2014
).
12.
Y.
Inokuchi
,
R.
Kusaka
,
T.
Ebata
,
O. V.
Boyarkin
, and
T. R.
Rizzo
,
ChemPhysChem
14
,
649
(
2013
).
13.
S.
Warnke
,
A.
Ben Faleh
,
R. P.
Pellegrinelli
,
N.
Yalovenko
, and
T. R.
Rizzo
,
Faraday Discuss.
217
,
114
(
2019
).
14.
S.-i.
Ishiuchi
,
H.
Wako
,
D.
Kato
, and
M.
Fujii
,
J. Mol. Spectrosc.
332
,
45
(
2017
).
15.
F. S.
Menges
,
E. H.
Perez
,
S. C.
Edington
,
C. H.
Duong
,
N.
Yang
, and
M. A.
Johnson
,
J. Am. Soc. Mass Spectrom.
30
,
1551
(
2019
).
16.
B. M.
Marsh
,
J. M.
Voss
, and
E.
Garand
,
J. Chem. Phys.
143
,
204201
(
2015
).
17.
N. C.
Polfer
and
J.
Oomens
,
Mass Spectrom. Rev.
28
,
468
(
2009
).
18.
A.
Günther
,
P.
Nieto
,
D.
Müller
,
A.
Sheldrick
,
D.
Gerlich
, and
O.
Dopfer
,
J. Mol. Spectrosc.
332
,
8
(
2017
).
19.
C. P.
Harrilal
,
A. F.
DeBlase
,
S. A.
McLuckey
, and
T. S.
Zwier
,
J. Phys. Chem. A
125
,
9394
(
2021
).
20.
O.
Asvany
and
S.
Schlemmer
,
Phys. Chem. Chem. Phys.
23
,
26602
(
2021
).
21.
S.
Spieler
,
M.
Kuhn
,
J.
Postler
,
M.
Simpson
,
R.
Wester
,
P.
Scheier
,
W.
Ubachs
,
X.
Bacalla
,
J.
Bouwman
, and
H.
Linnartz
,
Astrophys. J.
846
,
168
(
2017
).
22.
E.
Mucha
,
A. I.
González Flórez
,
M.
Marianski
,
D. A.
Thomas
,
W.
Hoffmann
,
W. B.
Struwe
,
H. S.
Hahm
,
S.
Gewinner
,
W.
Schöllkopf
,
P. H.
Seeberger
,
G.
von Helden
, and
K.
Pagel
,
Angew. Chem., Int. Ed.
56
,
11248
(
2018
).
23.
D. J.
Goebbert
,
G.
Meijer
, and
K. R.
Asmis
,
AIP Conf. Proc.
1104
,
22
(
2009
).
24.
S.
Chakrabarty
,
M.
Holz
,
E. K.
Campbell
,
A.
Banerjee
,
D.
Gerlich
, and
J. P.
Maier
,
J. Phys. Chem. Lett.
4
,
4051
(
2013
).
25.
O.
Dopfer
,
D.
Roth
, and
J. P.
Maier
,
Int. J. Mass Spectrom.
218
,
281
(
2002
).
26.
N.
Solcà
and
O.
Dopfer
,
J. Chem. Phys.
120
,
10470
(
2004
).
27.
K.
Koyasu
,
T.
Ohtaki
,
N.
Hori
, and
F.
Misaizu
,
Chem. Phys. Lett.
523
,
54
(
2012
).
28.
S.
Warnke
,
C.
Baldauf
,
M. T.
Bowers
,
K.
Pagel
, and
G.
von Helden
,
J. Am. Chem. Soc.
136
,
10308
(
2014
).
29.
A.
Masson
,
M. Z.
Kamrath
,
M. A. S.
Perez
,
M. S.
Glover
,
U.
Rothlisberger
,
D. E.
Clemmer
, and
T. R.
Rizzo
,
J. Am. Soc. Mass Spectrom.
26
,
1444
(
2015
).
30.
M. Z.
Kamrath
and
T. R.
Rizzo
,
Acc. Chem. Res.
51
,
1487
(
2018
).
31.
L.
Voronina
and
T. R.
Rizzo
,
Phys. Chem. Chem. Phys.
17
,
25828
(
2015
).
32.
B.
Schindler
,
A. D.
Depland
,
G.
Renois-Predelus
,
G.
Karras
,
B.
Concina
,
G.
Celep
,
J.
Maurelli
,
V.
Loriot
,
E.
Constant
,
R.
Bredy
,
C.
Bordas
,
F.
Lépine
, and
I.
Compagnon
,
Int. J. Ion Mobility Spectrom.
20
,
119
(
2017
).
33.
S. J. P.
Marlton
,
B. I.
McKinnon
,
B.
Ucur
,
J. P.
Bezzina
,
S. J.
Blanksby
, and
A. J.
Trevitt
,
J. Phys. Chem. Lett.
11
,
4226
(
2020
).
34.
O.
Hernandez
,
S.
Isenberg
,
V.
Steinmetz
,
G. L.
Glish
, and
P.
Maitre
,
J. Phys. Chem. A
119
,
6057
(
2015
).
35.
N. J. A.
Coughlan
,
W.
Fu
,
M.
Guna
,
B. B.
Schneider
,
J. C. Y.
Le Blanc
,
J. L.
Campbell
, and
W. S.
Hopkins
,
Phys. Chem. Chem. Phys.
23
,
20607
(
2021
).
36.
A.
Ben Faleh
,
S.
Warnke
, and
T. R.
Rizzo
,
Anal. Chem.
91
,
4876
(
2019
).
37.
B. D.
Adamson
,
N. J. A.
Coughlan
,
P. B.
Markworth
,
R. E.
Continetti
, and
E. J.
Bieske
,
Rev. Sci. Instrum.
85
,
123109
(
2014
).
38.
A.-L.
Simon
,
F.
Chirot
,
C. M.
Choi
,
C.
Clavier
,
M.
Barbaire
,
J.
Maurelli
,
X.
Dagany
,
L.
MacAleese
, and
P.
Dugourd
,
Rev. Sci. Instrum.
86
,
094101
(
2015
).
39.
R.
Fromherz
,
G.
Ganteför
, and
A. A.
Shvartsburg
,
Phys. Rev. Lett.
89
,
083001
(
2002
).
40.
M.
Vonderach
,
O. T.
Ehrler
,
P.
Weis
, and
M. M.
Kappes
,
Anal. Chem.
83
,
1108
(
2011
).
41.
G.
von Helden
,
M. T.
Hsu
,
N.
Gotts
, and
M. T.
Bowers
,
J. Phys. Chem.
97
,
8182
(
1993
).
42.
N. G.
Gotts
,
G.
von Helden
, and
M. T.
Bowers
,
Int. J. Mass Spectrom. Ion Processes
149–150
,
217
(
1995
).
43.
M. F.
Jarrold
and
J. E.
Bower
,
J. Chem. Phys.
96
,
9180
(
1992
).
44.
R. R.
Hudgins
,
M.
Imai
,
M. F.
Jarrold
, and
P.
Dugourd
,
J. Chem. Phys.
111
,
7865
(
1999
).
45.
P.
Weis
,
T.
Bierweiler
,
S.
Gilb
, and
M. M.
Kappes
,
Chem. Phys. Lett.
355
,
355
(
2002
).
46.
S.
Gilb
,
P.
Weis
,
F.
Furche
,
R.
Ahlrichs
, and
M. M.
Kappes
,
J. Chem. Phys.
116
,
4094
(
2002
).
47.
F.
Furche
,
R.
Ahlrichs
,
P.
Weis
,
C.
Jacob
,
S.
Gilb
,
T.
Bierweiler
, and
M. M.
Kappes
,
J. Chem. Phys.
117
,
6982
(
2002
).
48.
P.
Weis
,
O.
Welz
,
E.
Vollmer
, and
M. M.
Kappes
,
J. Chem. Phys.
120
,
677
(
2004
).
49.
J. W. J.
Wu
,
R.
Moriyama
,
M.
Nakano
,
K.
Ohshimo
, and
F.
Misaizu
,
Phys. Chem. Chem. Phys.
19
,
24903
(
2017
).
50.
K.
Ohshimo
,
T.
Komukai
,
R.
Moriyama
, and
F.
Misaizu
,
J. Phys. Chem. A
118
,
3899
(
2014
).
51.
E. K.
Campbell
and
P. W.
Dunk
,
Rev. Sci. Instrum.
90
,
103101
(
2019
).
52.
H. E.
Revercomb
and
E. A.
Mason
,
Anal. Chem.
47
,
970
(
1975
).
53.
K.
Kaplan
,
S.
Graf
,
C.
Tanner
,
M.
Gonin
,
K.
Fuhrer
,
R.
Knochenmuss
,
P.
Dwivedi
, and
H. H.
Hill
,
Anal. Chem.
82
,
9336
(
2010
).
54.
N. E.
Bradbury
and
R. A.
Nielsen
,
Phys. Rev.
49
,
388
(
1936
).
55.
X.-B.
Wang
and
L.-S.
Wang
,
Rev. Sci. Instrum.
79
,
073108
(
2008
).
56.
P.
Dugourd
,
R. R.
Hudgins
,
D. E.
Clemmer
, and
M. F.
Jarrold
,
Rev. Sci. Instrum.
68
,
1122
(
1997
).
57.
V.
Shrivastav
,
M.
Nahin
,
C. J.
Hogan
, and
C.
Larriba-Andaluz
,
J. Am. Soc. Mass Spectrom.
28
,
1540
(
2017
).
58.
B. H.
Besler
,
K. M.
Merz
, Jr.
, and
P. A.
Kollman
,
J. Comput. Chem.
11
,
431
(
1990
).
59.
P.
Freivogel
,
J.
Fulara
,
D.
Lessen
,
D.
Forney
, and
J. P.
Maier
,
Chem. Phys.
189
,
335
(
1994
).
60.
J.
Fulara
,
M.
Grutter
, and
J. P.
Maier
,
J. Phys. Chem. A
111
,
11831
(
2007
).
61.
J. T.
Buntine
,
M. I.
Cotter
,
U.
Jacovella
,
C.
Liu
,
P.
Watkins
,
E.
Carrascosa
,
J. N.
Bull
,
L.
Weston
,
G.
Muller
,
M. S.
Scholz
, and
E. J.
Bieske
,
J. Chem. Phys.
155
,
214302
(
2021
).
62.
M.
Förstel
,
K. M.
Pollow
,
K.
Saroukh
,
E. A.
Najib
,
R.
Mitric
, and
O.
Dopfer
,
Angew. Chem., Int. Ed.
59
,
21403
(
2020
).
63.
A.
Shayeghi
,
L. F.
Pašteka
,
D. A.
Götz
,
P.
Schwerdtfeger
, and
R.
Schäfer
,
Phys. Chem. Chem. Phys.
20
,
9108
(
2018
).
64.
M.
Förstel
,
W.
Schewe
, and
O.
Dopfer
,
Angew. Chem., Int. Ed.
131
,
3394
(
2019
).
65.
A.
Shayeghi
,
D. A.
Götz
,
R. L.
Johnston
, and
R.
Schäfer
,
Eur. Phys. J. D
69
,
152
(
2015
).
66.
A.
Shayeghi
,
R. L.
Johnston
, and
R.
Schäfer
,
J. Chem. Phys.
141
,
181104
(
2014
).