Conformation-selective resonant photoelectron imaging from dipole-bound states of cold 3-hydroxyphenoxide

The semiquinone radicals, HO(C₆H₄)O^⋅, are important electron transfer intermediates in biological processes such as photosynthesis and respiration.^1–3 Different from the o- and p-HO(C₆H₄)O^⋅ isomers, the 3-hydroxyphenoxy radical [m-HO(C₆H₄)O^⋅] is particularly interesting, because it can exist as two close-lying conformers, anti- and syn-m-HO(C₆H₄)O^⋅. The two conformations differ by the orientations of the hydrogen atom on the hydroxyl group,^4,5 as shown in Fig. 1, along with their respective precursor anions. The two conformers of the 3-hydroxyphenoxy radical have different dipole moments and can support dipole-bound states (DBS) with different binding energies (BEs).^6–9 In a previous preliminary study,¹⁰ we observed the two conformations in high-resolution photoelectron imaging (PEI) of cryogenically cooled 3-hydroxyphenoxide (3HOP) anions, m-HO(C₆H₄)O⁻. The electron binding energies of the two 3HOP conformations or the electron affinities of the anti- and syn-m-HO(C₆H₄)O^⋅ radicals were found to differ by 67 cm⁻¹ with the anti-conformer being slightly higher. More interestingly, the two conformers of the anion were found to possess excited DBS with different binding energies relative to the respective detachment thresholds, 490 cm⁻¹ and 104 cm⁻¹ for the anti- and syn-3HOP, respectively. Five near-threshold vibrational resonances were observed and conformation-selective PEI was realized via the vibrational levels of the DBS. In the current article, we report a more comprehensive investigation of the 3HOP anion in a wider spectral range, yielding extensive vibrational information for the two conformers of the m-HO(C₆H₄)O^⋅ radicals.

Molecular conformations are important in chemistry and biochemistry.^11–14 Several experimental techniques have been used to characterize conformational structures and dynamics, such as microwave spectroscopy,¹⁵ infrared (IR) spectroscopy,^13–16 and conformer-specific photodissociation dynamics and spectroscopy.^17–19 Conformers of neutral molecules with sufficiently different dipole moments can be selected using the Stark effects to probe conformation-dependent chemical reactivity.^20,21 High-resolution resonant photoelectron imaging via dipole-bound excited states of cold anions yields conformation-selective photoelectron spectra and can be considered as a new method to obtain conformation-selective spectroscopic information for dipolar molecular radicals.¹⁰ 

This method is based on mode-specific vibrational autodetachment from vibrational levels of DBS, first observed in cryogenically cooled phenoxide anions.^22,23 Due to the Δν=−1 propensity rule,^24,25 highly non-Franck-Condon resonant PE spectra were obtained to yield much more vibrational information, particularly for low-frequency and Franck-Condon-inactive vibrational modes.^26–32 Beyond the preliminary observation of five vibrational resonances due to the excited DBS of anti- and syn-3HOP,¹⁰ here we report photodetachment spectroscopy with photon energies up to ∼1000 cm⁻¹ above the detachment thresholds. A total of thirty vibrational resonances are observed, seventeen from the anti-3HOP conformer, eight from the syn-3HOP conformer, and five from overlapping vibrational levels of the DBS of both conformers. Conformation-selective resonant PE images have been obtained by tuning the detachment laser to the vibrational resonances. The photodetachment spectrum is assigned using the resonant PE spectra in conjunction with computed vibrational frequencies. Based on the photodetachment spectrum and resonant PE images, we are able to obtain fourteen fundamental vibrational frequencies for each of the m-HO(C₆H₄)O– conformations.

The experiment was done using our third-generation electrospray-photoelectron spectroscopy apparatus,²³ equipped with a cryogenically cooled Paul trap³³ and a high-resolution PE imaging system.³⁴ The 3HOP anions were produced by deprotonation of m-HO(C₆H₄)OH via electrospray of a 1 mM solution in a mixed solvent of CH₃OH/H₂O (9/1 in volume) at pH ∼ 10. Anions generated in the electrospray ionization source were guided into a cryogenically cooled Paul trap operated at 4.5 K. After being accumulated for 0.1 s and thermally cooled via collisions with 1 mTorr He/H₂ (4/1 in volume) background gas,^23,33 the anions were pulsed out at a 10 Hz repetition rate into the extraction zone of a time-of-flight mass spectrometer. The 3HOP anions were selected by a mass gate and photodetached in the interaction zone of the imaging lens by a dye laser. Photoelectrons were projected onto a pair of 75-mm diameter micro-channel plates coupled to a phosphor screen and captured by a charge-coupled device camera. The PE images were inverse-Abel transformed and reconstructed using the pBasex and BASEX programs.^35,36 The PE spectra were calibrated with the known spectra of Au^– at different photon energies. The kinetic energy (KE) resolution achieved was 3.8 cm⁻¹ for electrons with 55 cm⁻¹ KE and about 1.5% (ΔKE/KE) for KE above 1 eV in the current experiment.

Fig. 2 shows the non-resonant PE images and spectra of m-HO(C₆H₄)O⁻ at two photon energies. At 517.45 nm, three peaks, labeled as ^S0₀⁰, ^A0₀⁰, and A, are resolved. The superscripts “S” and “A” in the labels and throughout the text are used to designate the syn and anti conformations. As discussed in Ref. 10, peaks ^S0₀⁰ and ^A0₀⁰ at binding energies of 18 850 cm⁻¹ and 18 917 cm⁻¹ represent the vibrational origins of the syn- and anti-m-HO(C₆H₄)O^⋅ radical conformations or their electron affinities, respectively. It should be noted that vibrational hot bands are completely eliminated due to the effective vibrational cooling in the cryogenic ion trap.^{22,23,37–39} Peak A corresponds to the fundamental vibrational excitation of mode ^Sν₂₃ of syn-m-HO(C₆H₄)O^⋅. The relative high intensity of peak A is due to the near-threshold enhancement.¹⁰ At 501.01 nm, the normal Franck-Condon profiles are displayed for vibrational peaks far from threshold (the two near-threshold peaks L and M are enhanced). Peaks A–M correspond to transitions from the ground states of the anti- and syn-m-3HOP anions to different excited vibrational levels of the anti- and syn-m-HO(C₆H₄)O^⋅ neutral radicals. The electron binding energies of all the observed vibrational peaks, their shifts from the respective vibrational origins, and assignments are summarized in Table I, where the more accurate values are from the resonant PE spectra to be presented below.

Fig. 3 shows the photodetachment spectrum of 3HOP with photon energy ∼1000 cm⁻¹ above the detachment thresholds, labeled as ^SEA and ^AEA for the syn- and anti-3HOP conformers, respectively. This spectrum was obtained by monitoring the total electron yield while scanning the dye laser wavelength near and above the detachment thresholds of 3HOP.³⁷ The overall baseline above thresholds represents the cross section of direct non-resonant photodetachment of 3HOP. Thirty above-threshold resonances are observed and labeled as ^A1–^A17 (blue), ^S1–^S8 (red), and ^AS1–^AS5 (pink). These peaks indicate autodetachment from vibrational levels of the excited DBS of anti-3HOP, syn-3HOP, and overlapping levels of both conformers, respectively. The first five peaks (^A1–^A4 and ^S1) were reported in the previous preliminary study¹⁰ as well as the below-threshold vibrational origins of the DBS of the two conformers shown in the inset (^A0′ and ^S0′. Note a ′ sign is used to designate vibrational levels of the DBS). The two below-threshold origin peaks were very weak, because they were due to two-photon detachment. The ^A0′ and ^S0′ peaks defined the DBS binding energies of 490 and 104 cm⁻¹ for the anti- and syn-3HOP conformers, relative to their respective detachment thresholds (^AEA and ^SEA).¹⁰ The larger DBS binding energy of the anti-conformer is consistent with the larger dipole moment of the neutral radical (Fig. 1). The photon energies, shifts from the respective ground DBS vibrational levels, and assignments of the observed resonances are given in Table II. The assignments of all the vibrational peaks are based on the autodetachment in the resonant PE spectra shown in Figs. 4–6 and the calculated vibrational frequencies given in Table III.

By tuning the detachment laser to the above-threshold resonances in Fig. 3, we obtained thirty high-resolution resonantly enhanced PE images and spectra, as shown in Figs. 4–6. The detachment laser wavelength, the DBS vibrational level, and the corresponding peak label (in parentheses) used in the photodetachment spectrum of Fig. 3 are given in each resonant PE spectrum in Figs. 4–6. The resonant excitations involve different vibrational levels of the DBS of the two conformers of 3HOP, followed by autodetachment. The ten spectra in Fig. 4 contain autodetachment involving a single vibrational mode of the DBS, where the five spectra on the left, reported previously,¹⁰ are shown here for completeness and for comparison. The ten spectra in Fig. 5 represent excitations to combinational vibrational levels of the DBS, whereas the ten spectra in Fig. 6 correspond to excitations to overlapping vibrational levels of DBS of the two conformers. These resonant PE spectra involve two detachment channels:²² (1) non-resonant direct photodetachment represented by the baseline in Fig. 3 and (2) resonant autodetachment via the DBS represented by the resonances in Fig. 3. Due to mode-selectivity and the Δν = −1 propensity rule in the autodetachment process, highly non-Franck-Condon spectra are obtained, with certain vibrational levels significantly enhanced.^26–32 The enhanced vibrational levels in each spectrum are labeled in bold face. In addition, several vibrational peaks not observed in the non-resonant PE spectra in Fig. 2 appear in the resonant PE spectra in Figs. 5 and 6. They are labeled as a–h; their binding energies and assignments are also given in Table I.

To assist the assignment of the vibrational peaks in the PE spectra, we calculated the vibrational frequencies of anti- and syn-m-HO(C₆H₄)O^⋅ at the B3LYP/6-311++G(d,p) level, as shown in Table III. The vibrational modes and frequencies of the two conformers are very similar as expected, except the ν₃₁ mode, which corresponds to the out-of-plane rocking mode of the OH group (Fig. 7). Since both the anions and the neutral radicals of the two conformers are planar with C_s symmetry, only in-plane modes (A′) or even quanta of out-of-plane modes (A″) are allowed in principle in the PE spectra. The energy shifts of peaks A–M relative to the respective vibrational origins of the two conformers and their assignments are given in Table I. These peaks are due to vibrational excitations of either isomer or overlapping levels of both isomers. The assignments are accomplished by a careful comparison of the experimental shifts with the theoretical frequencies. For example, peak A has a shift of 357 cm⁻¹ relative to ^S0₀⁰ and 290 cm⁻¹ relative to ^A0₀⁰. The 290 cm⁻¹ shift does not agree with any frequencies of the vibrational modes of anti-m-HO(C₆H₄)O^⋅, while the 357 cm⁻¹ shift matches well the computed frequency for the in-plane ^Sν₂₃ (A′) mode (346 cm⁻¹ in Table III) of syn-m-HO(C₆H₄)O^⋅. Thus, peak A is assigned to the in-plane scissoring vibrational mode ^Sν₂₃ of syn-m-HO(C₆H₄)O^⋅. The energy shifts of peak B from ^S0₀⁰ and ^A0₀⁰ are 515 cm⁻¹ and 448 cm⁻¹, respectively. However, the 448 cm⁻¹ shift does not match any calculated frequency for anti-m-HO(C₆H₄)O^⋅, while the 515 cm⁻¹ shift can be readily assigned to the in-plane stretching mode ^Sν₂₁ (A′) (with a computed frequency of 509 cm⁻¹ in Table III) of syn-m-HO(C₆H₄)O^⋅, which is also confirmed by the autodetachment enhancement in the resonant PE spectra to be discussed below. Peak C, shifted from ^A0₀⁰ by 518 cm⁻¹, is assigned to the in-plane stretching mode ^Aν₂₁ (A′) (computed frequency of 511 cm⁻¹ in Table III) of anti-m-HO(C₆H₄)O^⋅. At higher binding energies, overlapping of vibrational levels from both conformers becomes possible, which makes the assignments difficult in some cases. Based on the autodetachment enhancement in the resonant PE spectra in Figs. 4–6, peaks D to J are assigned to ^A29¹ (A″), ^A28¹ (A″), ^S27¹ (A″), ^A27¹ (A″), ^S25¹ (A″), ^A25¹ (A″), and ^A18¹ (A′), respectively. Interestingly, except for the in-plane mode ^Aν₁₈ (A′), the other six modes are all out-of-plane modes (Aȃ), suggesting that the radical conformers may not be truly planar. In addition, peaks E and H can also be assigned to ^S23² (A′) and ^S21¹23¹, respectively, indicating overlapping levels. Peaks K to M are assigned to ^S17¹ (A′), ^S23³ (A′), and ^A21² (A′), respectively.

The dipole moments of anti- and syn-m-HO(C₆H₄)O^⋅ were calculated to be 5.3 D and 3.1 D, respectively (Fig. 1),¹⁰ which are larger than the 2.5 D practical critical dipole moment to support DBS.⁶ As reported in the previous preliminary study,¹⁰ the binding energies of the DBS are 490 cm⁻¹ and 104 cm⁻¹ for the anti- and syn-3HOP conformers relative to their respective detachment thresholds, as shown by the below-threshold detachment peaks ^A0′ and ^S0′ in the inset of Fig. 3. The thirty above-threshold peaks in Fig. 3 represent optical transitions to the excited vibrational levels of the DBS of anti- and syn-3HOP, followed by autodetachment.³⁷ The seventeen peaks, labeled as ^A1–^A17 (blue), are from vibrational levels of the DBS of the anti-3HOP conformer; the eight peaks, ^S1–^S8 (red), are from vibrational levels of syn-3HOP; and the five peaks, ^AS1–^AS5 (pink), are from overlapping vibrational levels of the DBS of both conformers. These assignments are done on the basis of the resonant PE spectra to be discussed below and by comparing the measured vibrational frequencies with those computed for the anti- and syn-conformers of the neutral radicals (Table III). The first five detachment peaks (^A1–^A4 and ^S1) were reported in the previous preliminary study.¹⁰ All the assignments given in Table II for these five peaks are the same, except the ^A1 peak, which is reassigned to the ^Aν₂₁′ mode, instead of the ^Aν₂₀′ mode assigned previously. This reassignment is done by considering the same mode in the more extensive resonant PE spectra to be discussed below. As shown previously,^27–32 the vibrational frequencies in the DBS of anions are the same as the corresponding neutral radicals within our experimental uncertainty, because the weakly bound excess electron in the DBS has negligible influence on the structures of the neutral cores. Since the peak width in Fig. 3 is mainly limited by rotational broadening,³⁷ the measured frequencies in the photodetachment spectrum are in general more accurate than those obtained from the PE spectra. The near-threshold PES features have comparable accuracy as in the photodetachment spectrum. The vibrational labels for the DBS are indicated by ′ in Table II. Most of the vibrational peaks observed in the PE spectra (Fig. 2 and Table I) are also observed in the photodetachment spectrum (Fig. 3 and Table II) with the same frequencies within the experimental accuracy.

It is interesting to note that the vibrational levels observed for the two conformers in the PE spectra in Fig. 2, as well as the photodetachment spectrum in Fig. 3, are not exactly the same, reflecting the slight different structures of the two conformers. In addition, more vibrational levels for the DBS of the anti-3HOP conformer are observed in the photodetachment spectrum, because of the broader excitation energy range as a result of the higher DBS binding energy of the anti-conformer, i.e., the lower excitation energy of the anti-conformer as defined by its 0–0 transition (^A0′ in Fig. 3).

Fig. 4 shows the resonant PE images and spectra of 3HOP that correspond to autodetachment from vibrational levels of single modes of the DBS for a given conformer. As shown previously,^28–32 autodetachment from vibrational levels of DBS generally follows the Δν=−1 propensity rule due to the similarity of the molecular structures in the DBS and the neutral,^24,25 that is, the nth vibrational level of a given mode (ν′_xⁿ) of the DBS is favored to autodetach to the (n − 1)th level of the corresponding neutral mode (ν_xⁿ⁻¹).²² The vibrationally induced autodetachment involves a strong vibronic coupling, during which one vibrational quantum of mode ν′_x is transferred to the DBS electron. Hence, only modes with sufficiently high frequencies can couple with the DBS electron for the vibrationally induced autodetachment. In comparison to the relative intensities of peaks ^S0₀⁰ and ^A0₀⁰ in the non-resonant spectra in Fig. 2, the ^A0₀⁰ peak in Figs. 4(a)–4(c) and 4(e)–4(h) is highly enhanced while the ^S0₀⁰ peak is almost negligible, indicating autodetachment from fundamental vibrational levels (^Aν′_x¹) of the DBS of the anti-m-HO(C₆H₄)O⁻ conformer. The assignments of Figs. 4(a)–4(e) have been discussed in the recent preliminary study,¹⁰ except one reassignment about the resonant peak (^A1) in Fig. 4(a), as mentioned above. The ^A1 peak in the photodetachment spectrum (Fig. 3) was assigned to the ^A20′¹ DBS level previously. However, on the basis of the PE spectra and the overtone excitation level in Fig. 4(j) (see below), the ^A1 peak excited in Fig. 4(a) should be due to the ^A21′¹ DBS level of the anti-3HOP conformer (Table II).

In Fig. 4(h), the detachment laser wavelength (515.83 nm) corresponds to the ^A7 resonant peak in the photodetachment spectrum (Fig. 3). The shift of the ^A7 peak relative to the ^A0′ DBS ground vibrational level of the anti-conformer is measured to be 959 cm⁻¹ (Table II), which is in good agreement with the computed frequency of the ν₂₄ (A′) mode of anti-m-HO(C₆H₄)O^⋅ (954 cm⁻¹, Table III). Thus, Fig. 4(h) corresponds to resonant excitation to the ^A24′¹ DBS vibrational level of the anti-3HOP conformer, followed by autodetachment to the ^A0₀⁰ ground state of the neutral anti-m-HO(C₆H₄)O^⋅ conformer according to the Δν = −1 propensity rule. Similarly, the PE spectra in Figs. 4(f) and 4(g) are due to autodetachment from the ^A26′¹ and ^A18′¹ DBS vibrational levels of the anti-3HOP conformer, respectively. The strong A peak in Fig. 4(f) is due to the near-threshold enhancement, similar to that observed in Fig. 2(a). The enhanced ^S0₀⁰ peak in Fig. 4(i) is due to autodetachment from the ^S28′¹ DBS vibrational level of the syn-3HOP conformer. In Fig. 4(j), the ^A21¹ vibrational level of the anti-m-HO(C₆H₄)O^⋅ conformer (peak C) is greatly enhanced. According to the Δν = −1 propensity rule, the enhanced peak C should be from autodetachment of the ^A21′² DBS vibrational level of the anti-3HOP conformer. Indeed, the photon energy used (513.61 nm) is shifted from the ^A0′ DBS ground state of the anti-3HOP conformer by 1043 cm⁻¹, which is exactly twice the frequency measured for the ^A21′ mode (Table II). Even though both the anti- and syn-3HOP conformers are present in the ion beam, the resonant excitation to specific vibrational levels via the DBS allows conformer-specific and autodetachment-enhanced resonant PE spectra to be obtained.

Autodetachment from combinational vibrational levels of DBS is more complicated because of multiple autodetachment channels. For example, excitation to a two-mode, above-threshold combinational DBS vibrational level (ν_x′^mν_y′ⁿ) can autodetach to a final neutral level of either ν_x^m−1ν_yⁿ or ν_x^mν_yⁿ⁻¹, following the Δν = −1 propensity rule and mode-selectivity. In Fig. 5(a), a single dominating peak f is observed at an electron binding energy of 19402 cm⁻¹ (Table I), which is not observed in the non-resonant PE spectrum in Fig. 2(b). Peak f has an energy shift of 485 cm⁻¹ from the ^A0₀⁰ origin peak of the anti-conformer, which is in good agreement with the computed frequency of the ν₂₂ (A′) mode of anti-m-HO(C₆H₄)O^⋅ (Table III). The photon energy used in Fig. 5(a) has a shift of 1004 cm⁻¹ to ^A0′, i.e., 485 cm⁻¹ + 519 cm⁻¹, corresponding to a combinational DBS vibrational level ^A21′¹22′¹ of the anti-3HOP conformer. The 519 cm⁻¹ vibrational frequency for the ^Aν₂₁′ mode deduced from the ^A8 resonant peak agrees well with that from the ^A1 resonant peak (522 cm⁻¹ in Table II). The frequency for the ^Aν₂₁′ DBS mode is the same as that of the ^Aν₂₁ mode measured in the PE spectra for the neutral radical (518 cm⁻¹ in Table I). In Fig. 5(a), only peak f (^A22¹) is enhanced because only one quantum of mode ^Aν₂₁′ (519 cm⁻¹) has enough energy to detach the DBS electron with a binding energy of 490 cm⁻¹. This is also the case in Fig. 5(b), where a combinational DBS vibrational level of ^A22′¹29′¹ is excited and only autodetachment to the final neutral level of ^A22¹ is energetically possible.

Figs. 5(d), 5(g), and 5(i) display cases involving excitation to two-mode combinational DBS levels of the anti-3HOP conformer, in which each mode can couple to the DBS electron to induce autodetachment. Hence, two enhanced peaks are observed in each spectrum. However, in Figs. 5(h) and 5(j), only one peak (I ^A25¹ in Fig. 5(h) and g ^A24¹ in Fig. 5(j)) is enhanced, even though both modes of the excited DBS level (^A21′¹25′¹ in Fig. 5(h) and ^A21′¹24′¹ in Fig. 5(j)) are energetically possible to induce autodetachment. This observation suggests that the coupling of the ^Aν₂₁′ mode with the DBS electron is much stronger than the ^Aν₂₄′ or ^Aν₂₅′ mode. Such mode-dependent vibronic coupling has been observed previously.^28–32 Figs. 5(c) and 5(e) show two more examples of mode-dependent vibrationally induced autodetachment. These two spectra involve resonant excitations to two-mode combinational DBS levels (^S23′¹30′¹ and ^S22′¹30′¹, respectively) of the syn-3HOP conformer. In both cases, only one vibrational peak is observed to be enhanced (A ^S23¹ in Fig. 5(c) and e ^S22¹ in Fig. 5(e)), even though the frequencies of both DBS modes in each case are energetically possible to induce autodetachment. In these cases, the mode that couples with the DBS electron is the ^Sν₃₀′ mode.

Fig. 5(f) shows a case of the breakdown of the Δν = −1 propensity rule. The ^S6 resonant peak (Fig. 3) corresponds to a three-mode combinational DBS vibrational level of the syn-3HOP conformer (^S23′¹31′¹33′¹ in Table II). However, only peak A (^S23¹) is enhanced in the resonant PE spectrum, suggesting the coupling of one quantum of ν₃₁′ and one quantum of ν₃₃′ simultaneously to the DBS electron during autodetachment. Such breakdown of the Δν = −1 propensity rule has been observed previously,^28–32 often involving low frequency DBS modes.

It is also worthwhile to point out that the resonantly enhanced peaks e, f, g in Fig. 5 due to autodetachment from combinational DBS vibrational levels represent new vibrational features that are not observed in the non-resonant PE spectra in Fig. 2, as shown in Table I. The weak peak c observed in Fig. 5(d) is also a new vibrational feature corresponding to the excitation of a low frequency bending mode (^Aν₃₂) of the anti-m-HO(C₆H₄)O^⋅ conformer.

For resonant excitations to overlapping DBS vibrational levels, the resonantly enhanced PE spectra can be even more complicated as a result of different autodetachment channels. Fig. 6 displays resonant PE spectra from overlapping DBS vibrational levels of three types: (1) those from the anti-3HOP conformer (Figs. 6(e) and 6(i)), (2) those from the syn-3HOP conformer (Figs. 6(c), 6(g), and 6(j)), and (3) those from both conformers (Figs. 6(a), 6(b), 6(d), 6(f), and 6(h)). The intensity ratio of the anti- and syn-conformers is defined by the ^S0₀⁰ and ^A0₀⁰ peaks in the non-resonant PE spectra shown in Fig. 2. Significant changes of the relative intensities of these two peaks in the resonant PE spectra indicate fundamental excitations of a specific DBS vibrational mode of a given conformer. In these cases, the fundamental vibrational frequency of a given DBS mode is degenerate with that of another mode or combinational modes. This happened in eight out of the ten spectra in Fig. 6. In Figs. 6(b), 6(e), and 6(i), the ^A0₀⁰ peak is enhanced relative to the ^S0₀⁰ peak, due to resonant excitations to the ^A25′¹, ^A14′¹, and ^A11′¹ DBS levels of the anti-3HOP conformer, respectively. The frequencies for the three modes are given in Tables II and III. In Figs. 6(a), 6(c), 6(d), 6(g), and 6(h), the ^S0₀⁰ peak is enhanced relative to the ^A0₀⁰ peak, due to excitations to the ^S22′¹, ^S29′¹, ^S26′¹, ^S18′¹, and ^S17′¹ of the syn-conformer, respectively, as shown in Tables II and III.

The overlapping combinational modes in Figs. 6(e) and 6(i) are also from the anti-conformer. In Fig. 6(e), both the C ^A21¹ and E ^A28¹ peaks are enhanced and the overlapping combinational mode can be straightforwardly assigned to the ^A21′¹28′¹ DBS level. In Fig. 6(i), the enhanced C ^A21¹ and h ^A26¹ peaks suggest excitation to the ^A21′¹26′¹ DBS level. Note that the h ^A26¹ peak was not resolved from the dominating H peak in the non-resonant PE spectrum (Fig. 2(b) and Table I). It should also be pointed out that the H peak is significantly enhanced in Fig. 6(i). However, there are no DBS levels that can autodetach to this level of the neutral syn-conformer. We attribute the enhanced H peak to the threshold effects, which are observed for the A peak (^S23¹) in a number of detachment wavelengths (for example, in Figs. 2(a) and 4(f)).

The overlapping combination modes in Figs. 6(c) and 6(g) are all from the syn-conformer. In Fig. 6(c), a new peak a ^S33¹ is observed, suggesting excitation to the ^S31′¹33′¹ combinational DBS level with only strong vibronic coupling by the ^Sν₃₁′ mode in the autodetachment. In Fig. 6(g), two peaks B ^S21¹ and F ^S27¹ are enhanced, suggesting two overlapping combinational DBS levels, ^S21′¹30′¹ and ^S27′¹33′¹, respectively, where again only one mode in each case is strongly coupled with the DBS electron for autodetachment. Fig. 6(j) also shows the same two enhanced peaks, B ^S21¹ and F ^S27¹, suggesting two overlapping combinational DBS levels, ^S21′¹28′¹ and ^S27′¹30′¹, respectively, also with only one mode in each case strongly coupled with the DBS electron.

The more complicated cases are those involving resonant excitations to overlapping DBS vibrational levels of both 3HOP conformers, as shown in Figs. 6(a), 6(b), 6(d), 6(f), and 6(h), corresponding to the resonant peaks ^AS1–^AS5 in Fig. 3. In Fig. 6(a), in addition to the enhanced ^S0₀⁰ peak due to the excitation to the ^S22′¹ DBS level of the syn-conformer, the c ^A32¹ peak is observed, suggesting an overlapping combinational DBS level of ^A30′¹32′¹33′¹ in violation of the Δν=−1 propensity rule, because vibrational quanta (^A30′¹33′¹) are coupled to the DBS electron for the autodetachment to the ^A32¹ final vibrational state. The strong peak A (^S23¹) is a result of threshold enhancement, as mentioned above. In Fig. 6(b), the enhanced peaks b ^A33¹ and A ^S23¹ imply excitation to an overlapping DBS level involving the two conformers, i.e., ^A21′¹33′²/^S23′¹33′¹. The enhanced D ^A29¹ peak in Fig. 6(d) suggests that the overlapping DBS level is ^A29′². In Fig. 6(f), the enhanced f ^A22¹ and B ^S21¹ peaks suggest overlapping combinational DBS levels of ^A22′¹27′¹/^S21′¹32′². Finally, in Fig. 6(h), the enhanced levels d ^S30¹ and G ^A27¹ imply two overlapping combinational DBS levels of ^S29′¹30′¹/^A27′¹29′¹, in addition to the overlapping ^S17′¹ DBS level. The strong H peak in Fig. 6(h) is attributed to threshold enhancement, similar to that in Fig. 6(i).

All the assignments for the observed DBS vibrational resonances in Fig. 3 are summarized in Table II. An energy level diagram is displayed in Fig. 8, showing the resonant excitations to the DBS vibrational levels of the two 3HOP conformers and the observed autodetachment channels. The electron detachment thresholds and the DBS binding energies for each conformer are also shown. Fig. 8 reveals the complexity of the 3HOP system due to the presence of the two conformers, as well as the wealth of spectroscopic information that can be obtained using resonant PES via the vibrational levels of the DBS. The experimental vibrational frequencies obtained for the two conformers of the m-HO(C₆H₄)O^⋅ radical are compared with the computed frequencies in Table III. In many cases, the same frequencies are measured from both the photodetachment spectrum of the DBS and the PE spectra (both resonant and non-resonant) for the neutral radicals. They are generally in agreement within the experimental uncertainties.

As shown in Fig. 1, the different orientation of the hydrogen atom on the hydroxyl group results in the two conformers of 3HOP. The intensity ratio of the 0–0 detachment transitions (^S0₀⁰ vs. ^A0₀⁰) in Fig. 2 suggests that the syn-conformer is more stable, consistent with a previous B3LYP calculation, which suggested that anti-3HOP is 0.855 kcal/mol higher in energy.⁵ Upon electron detachment, the corresponding neutral radicals, anti- and syn-m-HO(C₆H₄)O^⋅, are obtained and their electron affinities are different by 67 cm⁻¹ with the anti-conformer being slightly higher. Table III reveals that the vibrational frequencies of the neutral radicals of the two conformers are very similar, except mode ν₃₁. As shown in Fig. 7, this mode is the out-of-plane rocking mode of the –OH group and is related to the conversion of the two conformations. It is interesting to note that the frequency of this mode in the syn-conformer is much higher. At room temperature, these modes would be significantly populated, leading to equilibrium or free conversion of the two conformers. At lower temperatures, the two conformers are frozen out, but would not be separable. However, using resonant excitations to certain vibrational levels of the DBS, pure or nearly pure conformers of the neutral radicals can be produced. It was shown previously that resonant excitation to an intermediate electronic state of an anion allows different final neutral states to be accessed.⁴⁰ Fundamental excitations to single DBS vibrational modes of a given conformer can produce almost pure conformers of the corresponding neutral radical in its vibrational ground state according to the Δν = −1 autodetachment propensity rule. For example, using photodetachment wavelengths corresponding to those used in Figs. 4(a)–4(c), 4(e), and 4(g), one can produce almost pure neutral anti-m-HO(C₆H₄)O^⋅ radicals, whereas using the wavelength corresponding to that used for Fig. 4(d), one can produce almost pure neutral syn-m-HO(C₆H₄)O^⋅ radicals. A neutral conformer with specific vibrational energy can also be produced such as that in Fig. 4(j) or Fig. 5(a). Hence, resonant photodetachment may be a good technique to produce conformer-selected beams of neutral dipolar species for further investigations of conformation-dependent chemical and physical properties.

In conclusion, we report an extensive investigation of the photodetachment spectroscopy of cryogenically cooled m-HO(C₆H₄)O⁻ anions and resonant photoelectron imaging via vibrational levels of the dipole-bound excited states. Two conformers are observed for this anion and each possesses an excited dipole-bound state just below the detachment threshold. Thirty DBS vibrational resonances are observed for the two conformers with some of the resonances consisting of overlapping vibrational levels. Thirty resonant photoelectron spectra are obtained by tuning the detachment laser to each of the vibrational resonances. Extensive spectroscopic information is obtained for each of the conformers. The vibrational structures are assigned using computed frequencies of the two conformers and by using the resonant PE spectra on the basis of the propensity rule of vibrationally induced autodetachment from DBS. Resonant photoelectron spectroscopy of cold anions is shown to be an effective method to probe the spectroscopy of conformers of dipolar molecular species with sufficient dipole moments to support dipole-bound states.

We would like to thank Dr. Hong-Tao Liu and Dr. Chuan-Gang Ning for experimental assistance and valuable discussions. This work was supported by the National Science Foundation.