Excited state dynamics of molecules at interfaces can be studied using second-order non-linear spectroscopic methods such as time-resolved electronic sum-frequency generation (SFG). However, as such measurements inherently generate very small signals, they are often overwhelmed by signals originating from fluorescence. Here, this limitation is overcome by optical Kerr gating of the SFG signal to discriminate against fluorescence. The new approach is demonstrated on the excited state dynamics of malachite green at the water/air interface, in the presence of a highly fluorescent coumarin dye, and on the photo-oxidation of the phenolate anion at the water/air interface. The generality of the use of optical Kerr gating to SFG measurements is discussed.

Sum-frequency generation (SFG) is a non-linear optical process that occurs under intense electric fields and relies on the second-order electric susceptibility, χ(2), of materials. Within the electric dipole approximation, χ(2) is only non-zero where centrosymmetry is broken, and hence, SFG is restricted to non-centrosymmetric materials and interfaces over the length scale of the broken symmetry; this is the key property that enables its exploitation as a spectroscopic probe for materials and molecules at interfaces.1–4 Second-harmonic generation (SHG) is a subgroup of SFG (the two driving fields are of equal frequency), and when we discuss SFG, this may also include SHG in certain cases. The interest in SFG as a surface probe has grown over the past few decades5,6 as interfacial processes are ubiquitous in fields ranging from catalysis to biological and atmospheric chemistry.7–10 As the need to develop an understanding of the chemistry at interfaces increases, so too have the methods to experimentally probe this chemistry. Such methods include the development of time-resolved SFG,11,12 phase-sensitive SFG,13–16 heterodyne-detected SFG,17–19 broadband SFG spectroscopy,20–23 and 2D SFG.24 Common to many of these methods is their use of spectral ranges in the UV or IR regions, which each reveal complementary information about the chemical systems they probe. In recent years, there has been a notable shift toward the use of vibrational SFG (VSFG) spectroscopy, in which the SFG field at frequency ωSFG is generated by an IR field that is mixed with a near-IR one (typically 800 nm from a Ti:sapphire laser).25,26 The IR spectrum is then encoded through the enhancement of the SFG light when the IR field is resonant with vibrational transitions. VSFG has proven to be a powerful tool to analyze air–liquid interfaces and especially the nature of the air–water interface.27–30 Extension to phase-sensitive or heterodyne-detected VSFG (HD-VSFG) provides both the real and imaginary parts of χ(2), thus offering additional insight into aqueous interfaces. These methods have also been extended to the time domain, allowing the evolution of photo-induced dynamics of chromophores at aqueous interfaces to be probed in real time.31,32 However, by probing the interfacial solvent in such experiments, one does not directly probe the photo-dynamics of the chromophore, which is typically the species of interest. For example, it was recently shown that the dynamics of photoexcited phenol at the water–air interface differs by 4 orders of magnitude compared to similar dynamics of bulk water; however, this was inferred from the changes in the vibrational spectrum of the interfacial water molecules (i.e., the solvent), rather than by probing the evolution of the excited states of phenol or the production of products.33 

To probe photo-induced dynamics of molecules at interfaces directly, electronic sum-frequency generation (ESFG) can be used. In this case, ωSFG is generated by driving fields in the visible and/or near-UV. Similar to VSFG, it can be easily extended to the time domain (time-resolved, TR-ESFG) using a pump pulse and delayed probe fields to generate the SFG signal. Indeed, most early studies on the photo-induced dynamics of molecules at interfaces (especially aqueous) were TR-ESFG (or ESHG) experiments.12,34–37 Similar to VSFG, the ESFG signal can be resonantly enhanced through either the driving field or by ωSFG being resonant with an electronic transition in an interfacial chromophore. TR-ESFG can also be extended to be phase-sensitive38,39 and to heterodyne detection, as demonstrated in a series of impressive experiments by Tahara and co-workers.31,40 TR-ESFG is highly informative because, in principle, resonance enhancement of the SFG signal is proportional to the number of molecules on the surface, and hence, it can be used to track interfacial concentrations as a function of time. This is similar in spirit to transient absorption spectroscopy, which has become the workhorse to probe bulk excited state dynamics. Therefore, TR-ESFG has the potential to become a workhorse to track interfacial dynamics.

Despite the potential of ESFG as a general experimental method to probe interfacial photo-induced dynamics, it has experimental hurdles that are often not highlighted. As a χ(2) process, the SFG signal is generally weak compared to the driving fields. When detecting these weak signals, such as via photon counting in a simple homodyne-detected ESFG experiment, light generated from the surface that cannot be spectrally separated from ωSFG can result in poor signal-to-noise ratios. A particular issue arises in systems with fluorescent components that overlap with ωSFG in the frequency domain. Phase-sensitive measurements can alleviate this to some extent because of the incoherent nature of fluorescence, but it comes at the expense of increased experimental complexity, and there are limits to the extent of fluorescence that can be accommodated. Fluorescence is common and seen in a large fraction of chromophores that are of scientific interest, and because it can arise from the bulk and not just the surface, it generally dwarfs any SFG signal. Although not commonly acknowledged, the general approach to avoid the contribution of fluorescence is to ensure that ωSFG > ωpump, where ωpump is the frequency of the pump field that induces fluorescence. While effective, this approach is not always applicable because of the high density of states at high energy so that disentangling the transitions leading to resonance enhancement can be difficult. A potential alternative approach is to exploit the differences in the surface SFG vs fluorescence signals, where the former is coherent, polarized, and temporally short—on the time scale of the duration of the fundamental fields ω1 and ω2. As a result, optical methods could, in principle, be exploited to reject the fluorescence, which has previously been demonstrated in a similar fashion for Raman spectroscopy41 and time-resolved fluorescence spectroscopy.42–44 

In this study, we offer a simple and effective means to supress the fluorescence in a TR-ESFG experiment by using optical gating of the SFG signal. Specifically, we used optical Kerr gating (OKG) as a fast and broadband method. OKG is a common technique in ultrafast time-resolved fluorescence measurements, whereby a temporally long emission signal is gated for measurement in short windows set by an intense gate pulse.45,46 By scanning the gating window through a response, the entire signal can be built up to observe fluorescence with ∼100 fs resolution. OKG relies on an intense gate pulse inducing a change in refractive index in a material, the Kerr medium (KM), along one axis. When a linearly polarized “probe” field is transmitted through the KM without the gate present, its polarization remains unchanged on propagation. When the gate and probe are incident on the material at the same time, the probe will experience a different refractive index for its polarization components parallel and perpendicular to the gate, rotating the polarization of the probe. If the KM is set between two crossed polarizers, then the incoming light will not be transmitted unless it is rotated by the gate pulse. Therefore, in the context of surface ESFG, by timing the gate pulse to coincide with the SFG response from the surface, we anticipate that the probe will be transmitted through the KM, while the vast majority of the fluorescence emission, which lags the SFG, should be rejected. While TR-ESFG and OKG themselves are not new, the combination of these well-established methods provides a robust method to study excited state processes of molecules at interfaces.

The general layout of optical components for the time-resolved optical Kerr-gated ESFG (TR-OKG-ESFG) setup is shown in Fig. 1. All laser pulses used were derived from a Yb:KGW laser system (Carbide, Light Conversion) producing 84 μJ, 250 fs pulses at 1028 nm and with a 60 kHz repetition rate. ∼30 μJ pulse−1 was directed into a 514 nm pumped optical parametric amplifier (OPA) (Orpheus, Light Conversion). For the experiments described here, we used light at 720 nm (signal) with 1.3 μJ pulse−1. This served as one of the SFG driving fields, ω1. The remainder of the 1028 nm light (∼54 μJ pulse−1) was further split using a half-wave plate and polarizing beam-splitting optic to produce ∼4.1 μJ pulse−1, and this beam served as the second of the SFG driving fields, ω2. The remaining ∼50 μJ pulse−1 was used to generate pump pulses.

FIG. 1.

Simplified schematic of the experimental setup. PBS = polarizing beam splitter, λ/2 = half-wave plate, TS = translation stage, PC = Pockels cell, FHG = fourth harmonic generation, DO = dispersing optic (Pellin–Broca prism), L = lens, KM = Kerr medium, and PMT = photo-multiplier tube. Collinear beams are visually offset for clarity.

FIG. 1.

Simplified schematic of the experimental setup. PBS = polarizing beam splitter, λ/2 = half-wave plate, TS = translation stage, PC = Pockels cell, FHG = fourth harmonic generation, DO = dispersing optic (Pellin–Broca prism), L = lens, KM = Kerr medium, and PMT = photo-multiplier tube. Collinear beams are visually offset for clarity.

Close modal

The ω1 and ω2 beams were temporally overlapped using a manual delay line in the ω2 beam line and subsequently combined collinearly and focused onto the sample surface by a concave mirror (f = 20 cm) at ∼70° from the surface normal. Both the fundamental beams passed through appropriate half-wave plates prior to recombination, so their polarizations could be varied independently.

The pump beam was generated by first chopping the 1028 nm pulse train at 30 kHz using a rubidium titanyl phosphate (RTP) crystal (Leysop) and polarizing beam-splitting optic. This rotated the polarization of every other pulse in the train by 90° to be reflected and dumped. The remaining 30 kHz train of pulses was converted to its fourth harmonic using two consecutive β-barium borate (BBO) crystals, resulting in pulses at 257 nm, with ∼200 fs duration and up to 3 μJ pulse−1. This was aligned and focused onto the sample by a concave mirror (f = 20 cm) at ∼65° from the surface normal. An adjustable time delay, relative to the ω1 and ω2 pulses, was controlled by a commercial delay stage (Physik Instumente). The ESFG generated from the surface was collected in reflection geometry, and the residual reflected pump was blocked.

Sample solutions were contained in a standard petri dish, rotated at ∼15 rpm to refresh the sample. The height of the sample surface was monitored and maintained to within ±15 μm. After reflection from the sample surface, the beam containing ω1, ω2, and ωSFG (at 423 nm) was recollimated by a plano-convex lens (f = 25 cm), and ωSFG was separated from ω1 and ω2 using a Pellin–Broca prism.

The SFG was directed to the OKG arrangement. This consisted of two perpendicularly polarized Glan–Taylor prisms, between which were two plano-convex lenses (f = 15 cm and f = 5 cm, sequentially), and at the focus of the telescope, a UV fused silica cuvette with a 1 cm path length containing liquid benzene was placed. At the focus, the ωSFG beam has a diameter of <100 μm. Benzene was used as a Kerr medium because of its ease of use, good efficiency, and suitable response time.

To drive the OKG, residual 1028 nm light from the OPA was used (2.3 μJ pulse−1) and focused to a diameter of <100 μm by a plano-convex lens (f = 10 cm). The temporal delay between the probe and gate pulses was adjusted manually by a manual translation stage, and the polarization of the gate was set to 45° relative to the ωSFG using a half-wave plate.

The data acquisition involved using a photomultiplier tube (H7732-10, Hamamatsu) to convert ωSFG light into electrical pulses. These were then sent to an amplifier discriminator (F-100TD, Advanced Research Instruments Corp.), producing short transistor–transistor logic (TTL)-level pulses above a single-photon threshold. The output pulses were AND-gated with the synchronous output from the driving laser. To perform shot-to-shot background subtraction, the output TTL train was split into two separate 30 kHz channels by in-house electronics, each counted by a multifunction data acquisition (DAQ) device (USB-6210, National Instruments).

Experiments were conducted on malachite green (MG), malachite green with coumarin 2, and sodium phenolate. All chemicals were purchased from Sigma-Aldrich and used as received. Solutions were made up by dissolving the appropriate amount of chemical in high purity water (Millipore, Milli-Q Gradient A10, 18.2 MΩ). The coumarin dye was dissolved in ethanol. The sodium phenolate solution was made by combining equal parts of aqueous phenol and sodium hydroxide solutions.

To demonstrate the efficiency of the OKG, results on aqueous malachite green (75 μM) are first considered. Malachite green (MG) has been studied extensively in relation to aqueous surface nonlinear techniques on account of its strong nonlinear response and surface activity.36,47–49 The electronic structure of MG is characterized by a strong S1 ← S0 transition, peaking around 620 nm, and a second weaker S2 ← S0 transition, peaking around 420 nm. Excitation at 257 nm accesses a dense manifold of higher-lying excited states. Hence, ω1 and ω2 are not resonant with any transition, but ωSFG is expected to be resonant with the S2 ← S0 transition, and this resonance enhancement of the SFG signal is expected to be proportional to the square of the surface concentration.50 In Fig. 2, the square root of the SFG signal is shown for the dynamics probed over the first 40 ps following excitation. The SFG signal was obtained in PPP polarization configuration. The black trace shows the results when no OKG was applied, and the polarizers were set parallel to allow all light to pass. The signal at t < 0 arises from the SFG by the resonance enhancement with the S2 ← S0 transition. At t = 0, some population is transferred to higher-lying excited states, which reduces the concentration of molecules in the S0 state and, hence, reduces the SFG generated. The observed partial recovery shows that some population returns to S0. The time scale and overall dynamics are in excellent agreement with previous TR-SHG studies,48,51 including those from our group in which malachite green was excited at 250 nm and probed by 800 nm pulses with the SHG at 400 nm. The second, blue trace shows the results of an experiment in which the OKG was operated, and the two polarizers we set perpendicular to each other. The overall dynamics appear similar although there is a small overall reduction in the signal. To directly compare the two traces shown in Fig. 2(b), the difference in the square root of the SFG signal is shown, in which the signal from t < 0 has been subtracted to leave only the dynamical changes.

FIG. 2.

(a) Transient surface SFG signal (PPP) recorded from aqueous MG without OKG (black) and with OKG (blue). (b) Background-subtracted traces of the same data in (a). The inset shows the efficiency of the Kerr gate as a function of delay between the gate and SFG pulses (dots) with the Gaussian fit (green line). Positive gate delay indicates that the SFG pulse arrives after the gate. (c) Transient surface SFG signal (PPP) recorded from aqueous MG without OKG (black) and with OKG (blue), in the presence of coumarin 2.

FIG. 2.

(a) Transient surface SFG signal (PPP) recorded from aqueous MG without OKG (black) and with OKG (blue). (b) Background-subtracted traces of the same data in (a). The inset shows the efficiency of the Kerr gate as a function of delay between the gate and SFG pulses (dots) with the Gaussian fit (green line). Positive gate delay indicates that the SFG pulse arrives after the gate. (c) Transient surface SFG signal (PPP) recorded from aqueous MG without OKG (black) and with OKG (blue), in the presence of coumarin 2.

Close modal

As seen clearly in Fig. 2(b), the overall appearance of the kinetics is unaffected (as expected). The small differences in amplitude arise from the small changes in alignment required when rotating one of the polarizers. From direct measurements of the photomultiplier output seen from the square root of signals before t0 in Fig. 2(a), we determine that the efficiency of the OKG is ∼75%. Note that each time-resolved trace in Fig. 2 required only 5 min of data acquisition.

The inset in Fig. 2(b) shows the temporal resolution of the OKG. This was measured by operating the OKG set to achieve maximal transmission of a ωSFG pulse generated in a BBO crystal above the surface as a function of delay between the gate and the ωSFG pulses, then scanning the delay, and optimizing the overlap between the gate pulse and the ωSFG pulse. The inset of Fig. 2(b) clearly demonstrates that the ωSFG pulse is only observable in the presence of the gate. The overall response of the transmission closely follows that documented previously for benzene, which predominantly displays a Gaussian, electronic response, with a visible asymmetry over a few picoseconds resulting from the slower, nuclear response of the liquid.45,52

Figures 2(a) and 2(b) provide the demonstration of the function of OKG and its efficiency. However, malachite green does not exhibit much fluorescence, even when pumped at 257 nm. To test the impact of a fluorophore, a highly fluorescent dye molecule was added to the malachite green solution. Specifically, the addition of coumarin 2 will lead to strong fluorescence at 420 nm and may be expected to saturate the photon counter. To enable this measurement, we reduced the intensity of the ω2 field, and by extension that of ωSFG, so as to simulate a more characteristic strength of the surface response. It must also be noted that the pump intensity was decreased for this measurement, as otherwise the surface fluorescence alone was enough to saturate the pump-on counter. Without the addition of OKG, the fluorescence from the sample is much greater than that of the SFG signal from MG, and even the relatively large signal from MG is obscured by the fluorescence when photon counting in this regime. However, the fluorescence occurs over much longer time scales than the SFG signal, and by aligning the OKG with the SFG pulse, the vast majority of the photons generated by fluorescence, which will lag the SFG by a range of time up to nanoseconds, can be suppressed. This is shown in Fig. 2(c), where the ungated signal (black trace) is virtually featureless, whereas the OKG signal (blue trace) shows a distinct change after t0, albeit with a reduced signal-to-noise ratio on account of the reduced pump and ωSFG intensities. These traces also comprise a single scan (0.5 s acquisition time per data point) for comparison. Even in this extreme case of very strong fluorescence, application of the OKG allows the TR-ESFG signals to be acquired.

Finally, the utility of OKG applied to surface TR-ESFG spectroscopy is supported by measurements on the phenolate anion. Phenol and its anion have been studied previously by SHG and SFG.33,53,54 Additionally, the photo-induced dynamics have been monitored in the bulk by transient absorption55,56 and very recently at the surface using TR-HD-VSFG for phenol. It has been shown in the bulk solution that excitation of phenolate into the S1 state at 257 nm will produce hydrated electrons and phenoxy radicals, the former of which is resonant with ω1. Thus, following excitation, the SFG signal is expected to increase, with signal components that are approximately proportional to the square of the concentrations of hydrated electrons at the surface. At 0.1M bulk concentration, despite a low quantum yield of 0.007 in aqueous solution,57 the fluorescence from the phenolate molecules was large compared to the SFG response, completely saturating the photon counter and preventing observation of the interfacial kinetics. At reduced pump intensities and without OKG, the surface signal is too weak to be detected. Hence, the need for some form of discrimination against fluorescence is clearly highlighted in this system.

Figure 3 shows the results of TR-OKG-ESFG of the phenolate anion at the water/air interface following excitation at 257 nm (with SFG acquired in the PPP configuration). By applying OKG to the SFG signal, the fluorescence can be effectively removed from the detected signal. The data in Fig. 3 are an average of two scans, equating to 15 min of data acquisition. The positive signal at t > 0 indicates an increased concentration of hydrated electrons following pump excitation at the aqueous surface. The signal rises to a maximum at ∼2.5 ps and subsequently decays on a time scale of ∼20 ps, leaving a long-lived offset at later times. Similar overall dynamics have been observed in the bulk, albeit with slightly different time scales. The initial rise in the signal was associated with the formation of a contact pair of the phenoxy radical and a solvated electron. The subsequent decay was assigned to a competition between geminate recombination to reform the phenolate anion and dissociation of the contact pair to form free hydrated electrons. The latter can then be assigned to the observed offset at long times. This subsequently decays in the bulk due to diffusion of the phenoxy radical and hydrated electron that can subsequently recombine. The overall similarity between bulk and interfacial dynamics suggests that a very similar process takes place at the surface, although subtly different solvation environments might affect the lifetimes.39,58 This will be investigated in detail in a later study.

FIG. 3.

Transient surface SFG signal (PPP) recorded from 0.1M aqueous sodium phenolate with OKG.

FIG. 3.

Transient surface SFG signal (PPP) recorded from 0.1M aqueous sodium phenolate with OKG.

Close modal

To estimate the efficiency of fluorescence rejection by the Kerr gating, we monitored the photon count rate with no probe pulses (ω1 or ω2) present. In the presence of only pump pulses and with the OKG operating, the maximum photon count rate was 1.3 × 10−4 counts pulse−1. With gate polarizers in parallel, the OKG gate pulse is blocked, and the light attenuated to 10% transmission (using a OD1 neutral density filter), and the maximum photon count rate was 0.4 counts pulse−1. From this, we estimate that the Kerr gate transmits less than one in every 30 000 fluorescence photons for phenolate pumped at 257 nm.

OKG has the potential to be widely applicable to many surface SFG setups: it is simple to incorporate provided a sufficiently intense gate pulse can be supplied to facilitate a high gating efficiency. The technique can also be used in phase-sensitive SFG experiment that makes use of spatial interference or phase-delay methods, where transmission of a single interfering signal pulse through the gate is straightforward. Conversely, OKG may be less suited for heterodyne-detected methods in which the local oscillator and SFG fields are separated by a few picoseconds in time to generate a spectral interferogram, unless the OKG gate can be opened for a sufficient time (at the expense of increased fluorescence contributions). Kerr gating is not wavelength specific and is, therefore, pertinent to broadband ESFG and ESHG, enabling its application to many existing techniques. OKG is a well-established technique that has a broad range of literature on the behaviors of different materials used as a KM for different applications. By combining SFG techniques and OKG in detection, a large range of interesting surface-active compounds can be made accessible for surface nonlinear spectroscopy that may previously have been considered off-limits.

The authors are grateful to Professor Andy Beeby for many useful discussions concerning OKG and the loan of Glan–Taylor prisms. C.J.C.J. was supported by the ESPRC Doctoral Training Partnership (Grant No. EP/R513039/1).

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.
N.
Bloembergen
and
P. S.
Pershan
,
Phys. Rev.
128
,
606
(
1962
).
2.
P.
Guyot-Sionnest
,
W.
Chen
, and
Y. R.
Shen
,
Phys. Rev. B
33
,
8254
(
1986
).
3.
Y. R.
Shen
,
Principles of Nonlinear Optics
(
Wiley-Interscience
,
2002
).
5.
6.
7.
C.
George
,
M.
Ammann
,
B.
D’Anna
,
D. J.
Donaldson
, and
S. A.
Nizkorodov
,
Chem. Rev.
115
,
4218
(
2015
).
8.
C. T.
Williams
and
D. A.
Beattie
,
Surf. Sci.
500
,
545
(
2002
).
9.
O.
Björneholm
,
M. H.
Hansen
,
A.
Hodgson
,
L.-M.
Liu
,
D. T.
Limmer
,
A.
Michaelides
,
P.
Pedevilla
,
J.
Rossmeisl
,
H.
Shen
,
G.
Tocci
,
E.
Tyrode
,
M.-M.
Walz
,
J.
Werner
, and
H.
Bluhm
,
Chem. Rev.
116
,
7698
(
2016
).
10.
S.
Yang
,
H.
Noguchi
, and
K.
Uosaki
,
J. Phys. Chem. C
122
,
8191
(
2018
).
11.
Y.
Rao
,
D.
Song
,
N. J.
Turro
, and
K. B.
Eisenthal
,
J. Phys. Chem. B
112
,
13572
(
2008
).
12.
A.
Castro
,
E. V.
Sitzmann
,
D.
Zhang
, and
K. B.
Eisenthal
,
J. Phys. Chem.
95
,
6752
(
1991
).
13.
R. K.
Chang
,
J.
Ducuing
, and
N.
Bloembergen
,
Phys. Rev. Lett.
15
,
6
(
1965
).
14.
R.
Superfine
,
J. Y.
Huang
, and
Y. R.
Shen
,
Opt. Lett.
15
,
1276
(
1990
).
15.
K.
Kemnitz
,
K.
Bhattacharyya
,
J. M.
Hicks
,
G. R.
Pinto
,
B.
Eisenthal
, and
T. F.
Heinz
,
Chem. Phys. Lett.
131
,
285
(
1986
).
16.
R.
Stolle
,
G.
Marowsky
,
E.
Schwarzberg
, and
G.
Berkovic
,
Appl. Phys. B: Lasers Opt.
63
,
491
(
1996
).
17.
I. V.
Stiopkin
,
H. D.
Jayathilake
,
A. N.
Bordenyuk
, and
A. V.
Benderskii
,
J. Am. Chem. Soc.
130
,
2271
(
2008
).
18.
S.
Yamaguchi
and
T.
Tahara
,
J. Chem. Phys.
129
,
101102
(
2008
).
19.
S.
Sun
,
R.
Liang
,
X.
Xu
,
H.
Zhu
,
Y. R.
Shen
, and
C.
Tian
,
J. Chem. Phys.
144
,
244711
(
2016
).
20.
J. A.
McGuire
,
W.
Beck
,
X.
Wei
, and
Y. R.
Shen
,
Opt. Lett.
24
,
1877
(
1999
).
21.
E. W. M.
van der Ham
,
Q. H. F.
Vrehen
, and
E. R.
Eliel
,
Opt. Lett.
21
,
1448
(
1996
).
22.
L. J.
Richter
,
T. P.
Petralli-Mallow
, and
J. C.
Stephenson
,
Opt. Lett.
23
,
1594
(
1998
).
23.
K.
Sekiguchi
,
S.
Yamaguchi
, and
T.
Tahara
,
J. Chem. Phys.
128
,
114715
(
2008
).
24.
W.
Xiong
,
J. E.
Laaser
,
R. D.
Mehlenbacher
, and
M. T.
Zanni
,
Proc. Natl. Acad. Sci. U. S. A.
108
,
20902
(
2011
).
25.
X. D.
Zhu
,
H.
Suhr
, and
Y. R.
Shen
,
Phys. Rev. B
35
,
3047
(
1987
).
26.
S.
Yamaguchi
and
T.
Otosu
,
Phys. Chem. Chem. Phys.
23
,
18253
(
2021
).
27.
M.
Vinaykin
and
A. V.
Benderskii
,
J. Phys. Chem. Lett.
3
,
3348
(
2012
).
28.
N.
Ji
,
V.
Ostroverkhov
,
C. S.
Tian
, and
Y. R.
Shen
,
Phys. Rev. Lett.
100
,
096102
(
2008
).
29.
C.-S.
Tian
and
Y. R.
Shen
,
J. Am. Chem. Soc.
131
,
2790
(
2009
).
30.
S.
Nihonyanagi
,
R.
Kusaka
,
K. I.
Inoue
,
A.
Adhikari
,
S.
Yamaguchi
, and
T.
Tahara
,
J. Chem. Phys.
143
,
124707
(
2015
).
31.
K.
Matsuzaki
,
R.
Kusaka
,
S.
Nihonyanagi
,
S.
Yamaguchi
,
T.
Nagata
, and
T.
Tahara
,
J. Am. Chem. Soc.
138
,
7551
(
2016
).
32.
K.-i.
Inoue
,
T.
Ishiyama
,
S.
Nihonyanagi
,
S.
Yamaguchi
,
A.
Morita
, and
T.
Tahara
,
J. Phys. Chem. Lett.
7
,
1811
(
2016
).
33.
R.
Kusaka
,
S.
Nihonyanagi
, and
T.
Tahara
,
Nat. Chem.
13
,
306
(
2021
).
34.
R.
Antoine
,
A. A.
Tamburello-Luca
,
P.
Hébert
,
P. F.
Brevet
, and
H. H.
Girault
,
Chem. Phys. Lett.
288
,
138
(
1998
).
35.
D.
Zimdars
,
J. I.
Dadap
,
K. B.
Eisenthal
, and
T. F.
Heinz
,
J. Phys. Chem. B
103
,
3425
(
1999
).
36.
X.
Shi
,
E.
Borguet
,
A. N.
Tarnovsky
, and
K. B.
Eisenthal
,
Chem. Phys.
205
,
167
(
1996
).
37.
J. A.
McGuire
and
Y. R.
Shen
,
Science
313
,
1945
(
2006
).
38.
A. L.
Tyson
,
D. A.
Woods
, and
J. R. R.
Verlet
,
J. Chem. Phys.
149
,
204201
(
2018
).
39.
P. J.
Nowakowski
,
D. A.
Woods
, and
J. R. R.
Verlet
,
J. Phys. Chem. Lett.
7
,
4079
(
2016
).
40.
S.
Yamaguchi
and
T.
Tahara
,
J. Phys. Chem. C
119
,
14815
(
2015
).
41.
P.
Matousek
,
M.
Towrie
,
C.
Ma
,
W. M.
Kwok
,
D.
Phillips
,
W. T.
Toner
, and
A. W.
Parker
,
J. Raman Spectrosc.
32
,
983
(
2001
).
42.
J.
Takeda
,
K.
Nakajima
,
S.
Kurita
,
S.
Tomimoto
,
S.
Saito
, and
T.
Suemoto
,
Phys. Rev. B
62
,
10083
(
2000
).
43.
H.
Lemmetyinen
,
N. V.
Tkachenko
,
B.
Valeur
,
J.-i.
Hotta
,
M.
Ameloot
,
N. P.
Ernsting
,
T.
Gustavsson
, and
N.
Boens
,
Pure Appl. Chem.
86
,
1969
(
2014
).
44.
T.
Fujino
,
T.
Fujima
, and
T.
Tahara
,
Appl. Phys. Lett.
87
,
131105
(
2005
).
45.
S.
Arzhantsev
and
M.
Maroncelli
,
Appl. Spectrosc.
59
,
206
(
2005
).
46.
K.
Appavoo
and
M. Y.
Sfeir
,
Rev. Sci. Instrum.
85
,
055114
(
2014
).
47.
P.
Fita
,
A.
Punzi
, and
E.
Vauthey
,
J. Phys. Chem. C
113
,
20705
(
2009
).
48.
P. J.
Nowakowski
,
D. A.
Woods
,
C. D.
Bain
, and
J. R. R.
Verlet
,
J. Chem. Phys.
142
,
084201
(
2015
).
49.
A.
Punzi
,
G.
Martin-Gassin
,
J.
Grilj
, and
E.
Vauthey
,
J. Phys. Chem. C
113
,
11822
(
2009
).
50.
J. M.
Hicks
,
K.
Kemnitz
,
K. B.
Eisenthal
, and
T. F.
Heinz
,
J. Phys. Chem.
90
,
560
(
1986
).
51.
P.
Sen
,
S.
Yamaguchi
, and
T.
Tahara
,
Faraday Discuss.
145
,
411
(
2010
).
52.
Y.
Wang
,
L.
Sui
,
Y.
Jiang
,
D.
Liu
,
Q.
Li
,
A.
Chen
, and
M.
Jin
,
Proc. SPIE
10173
,
101731Y
(
2017
).
53.
A. A.
Tamburello-Luca
,
P.
Hébert
,
P. F.
Brevet
, and
H. H.
Girault
,
J. Chem. Soc., Faraday Trans.
92
,
3079
(
1996
).
54.
R.
Kusaka
,
T.
Ishiyama
,
S.
Nihonyanagi
,
A.
Morita
, and
T.
Tahara
,
Phys. Chem. Chem. Phys.
20
,
3002
(
2018
).
55.
A. L.
Tyson
and
J. R. R.
Verlet
,
J. Phys. Chem. B
123
,
2373
(
2019
).
56.
X.
Chen
,
D. S.
Larsen
,
S. E.
Bradforth
, and
I. H. M.
Van Stokkum
,
J. Phys. Chem. A
115
,
3807
(
2011
).
57.
N.
Getoff
,
Int. J. Radiat. Appl. Instrum.
34
,
711
(
1989
).
58.
D. M.
Sagar
,
C. D.
Bain
, and
J. R. R.
Verlet
,
J. Am. Chem. Soc.
132
,
6917
(
2010
).
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