Resonant Multiphoton Ionization Research Articles

Sub-PPB Detection with Gas-Phase Multiphoton Electron Extraction Spectroscopy under Ambient Conditions.

Multiphoton electron extraction spectroscopy (MEES) is an advanced analytical technique that has demonstrated exceptional sensitivity and specificity for detecting molecular traces on solid and liquid surfaces. Building upon the solid-state MEES foundations, this study introduces the first application of MEES in the gas phase (gas-phase MEES), specifically designed for quantitative detection of gas traces at sub-part per billion (sub-PPB) concentrations under ambient atmospheric conditions. Our experimental setup utilizes resonant multiphoton ionization processes using ns laser pulses under a high electrical field. The generated photoelectron charges are recorded as a function of the laser's wavelength. This research showcases the high sensitivity of gas-phase MEES, achieving high spectral resolution with resonant peak widths less than 0.02 nm FWHM. We present results from quantitative analysis of benzene and aniline, two industrially and environmentally significant compounds, demonstrating linear responses in the sub-PPM and sub-PPB ranges. The enhanced sensitivity and resolution of gas-phase MEES offer a powerful approach to trace gas analysis, with potential applications in environmental monitoring, industrial safety, security screening, and medical diagnostics. This study confirms the advantages of gas-phase MEES over many traditional optical spectroscopic methods and demonstrates its potential in direct gas-trace sensing in ambient atmosphere.

Hydration Rearrangement in the 4-Aminobenzonitrile-(H2 O)2 Cluster Induced by Photoionization: The Effect of Solvent-Solvent Interactions.

The interplay between solute-solvent and solvent-solvent interactions plays an essential role in solvation dynamics that has important effects on the mechanism and dynamics of chemical reactions in solution. In this study, the rearrangement of the hydration shell induced by photoionization of a solute molecule is probed in a state- and isomer-specific manner by resonant multiphoton ionization detected IR spectroscopy of the prototypical 4-aminobenzonitrile-(H2 O)2 cluster produced in a molecular beam. IR spectra reveal that the water molecules form a cyclic solvent network around the CN group in the initial neutral state (S0 ). Different from the singly-hydrated cluster, in which either the CN or the NH2 group is hydrated, hydration of the NH2 group is not observed in the dihydrated cluster. IR spectra obtained after ionizing the solute molecule into the cation ground state (D0 ) exhibit features ascribed to both NH-bound and CN-bound isomers, indicating that water molecules migrate from the CN to the NH site upon ionization with a yield depending on the ionization excess energy. Analysis of the IR spectra as a function of the excess energy shows that migration produces two different NH2 solvated structures, namely (i) the most stable structure in which both N-H bonds are singly hydrated and (ii) the second most stable isomer in which one of the N-H bonds is hydrated by a H-bonded (H2 O)2 dimer. The product branching ratio of the two isomers depends on the excess energy. The role of the water-water interaction in the hydration rearrangement is discussed based on the potential energy landscape. Solvation dynamics plays an important role in reaction mechanisms in the condensed phase, where not only solute-solvent solvation but also solvent-solvent interactions have a significant influence on the dynamics. Thus, the investigation of solvation dynamics at the molecular level substantially contributes to our understanding of the reaction mechanism. In this study, the dihydrated cluster of 4ABN was utilized as a model for the first solvation layer to elucidate solvent motions induced by ionization of the solute and the role of W-W interactions for the solvent relaxation.

Strong-field ionization and AC-Stark shifted Rydberg states in OCS

We present theoretical results for intensity-dependent above-threshold ionization (ATI) spectra from oriented OCS molecules probed by intense femtosecond laser pulses with wavelengths of 800 and 400 nm. The calculations were performed using the time-dependent Schrödinger equation within the single-active-electron approximation and including multielectron polarization effects. The results capture some of the experimental findings (Yu et al 2017 J. Phys. B: At. Mol. Opt. Phys. 50 235602), in particular when considering the sensitivity of the ATI spectra on the molecular orientation. We identify characteristic features in the ATI spectra which correspond to resonant multiphoton ionization via highly-excited Rydberg states are captured by the theory.

Nuclear quantum dynamics on the ground electronic state of neutral silver dimer 107Ag109Ag probed by femtosecond NeNePo spectroscopy.

The nuclear quantum dynamics on the ground electronic state of the neutral silver dimer 107Ag109Ag are studied by femtosecond (fs) pump-probe spectroscopy using the 'negative ion - to neutral - to positive ion' (NeNePo) excitation scheme. A vibrational wave packet is prepared on the X1Σ+g state of Ag2via photodetachment of mass-selected, cryogenically cooled Ag2- using a first ultrafast pump laser pulse. The temporal evolution of the wave packet is then probed by an ultrafast probe pulse via resonant multiphoton ionization to Ag2+. Frequency analysis of the fs-NeNePo spectra obtained for a single isotopologue and pump-probe delay times up to 60 ps yields the harmonic (ωe = 192.2 cm-1), quadratic anharmonic (ωexe = 0.637 cm-1) and cubic anharmonic (ωeye = 3 × 10-4 cm-1) constants for the X1Σ+g state of neutral Ag2. The fs-NeNePo spectra obtained at different pump wavelengths provide insight into the excitation mechanism. At a pump wavelength of 510 nm instead of 1010 nm, resonant excitation of a short-lived electronically excited state of the anion followed by autodetachment results in population of higher-energy vibrational levels of the neutral ground state. In contrast, at 1140 nm dynamics with a slightly shorter beating period and different relative phase are observed. The present study demonstrates that isotopologue-specific fs-NeNePo spectroscopy provides accurate vibrational constants of mass-selected neutral clusters in their electronic ground state in the terahertz spectral region, which remains difficult to obtain directly in the frequency domain with any other type of spectroscopy of comparable sensitivity.

Pushing multiphoton resonant ionization of the argon atom to the low-intensity regime

Resonance-enhanced multiphoton ionization process of the argon atom by an 800-nm 30-fs linearly polarized laser field is investigated at intensities range from 1.1 to $4.55\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$. At $4.55\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$ intensity the experimental photoelectron energy spectrum is in a good agreement with the time-dependent Schr\"odinger equation (TDSE) calculation where the double structure originating from dressed $4p\ensuremath{-}4d$ coupled transition is clearly identified. At lower intensity of $1.1\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$, the resonant ionization process via the $4f$ state is observed, however, the expected peak (jet) at ${90}^{\ensuremath{\circ}}$ in the photoelectron angular distribution from the zeroth order above threshold ionization, has vanished completely. Such behavior is attributed to the destructive interference phenomenon in the coherent contributions of different partial waves of the photoelectron, namely, $\ensuremath{\epsilon}d$ and $\ensuremath{\epsilon}g$ states, and has been confirmed in our TDSE calculations.

Intrinsic resonant photoionization time delay of hydrogen atoms probed with attosecond beating of asymmetrical photon transitions

We investigate the benchmark photoionization of hydrogen atoms triggered by the third harmonic at vacuum ultraviolet laser pulses (at 202 nm) and probe the time delay via resonant quantum states with the weak fundamental laser pulses (at 606 nm). As a result, two different sidebands will emerge below the first above threshold ionization peak. We show that these two sidebands can be established through resonance with the $3d$ and $2p$ states of hydrogen atoms at 202 nm, respectively. Using this scheme, we can extract the angularly dependent time delay of electrons liberated from the $3d$ state with respect to those released from the $2p$ state by comparing the phase differences between two sidebands. Using the attosecond beating scheme of asymmetrical photon transition, we obtain the intrinsic time delay between the $3d$ and $2p$ excited electronic states in the few-photon ionization regime of H atoms, which is \ensuremath{\sim}103 as. This asymmetric attosecond beating metrology provides insight into electron dynamics of the multiphoton resonant ionization via multi-intermediate states.

Ultrafast dynamics of acetone photooxidation on TiO2(110).

Using light energy to drive chemical reactions on semiconductor surfaces is the basis for technological applications ranging from the removal of organic pollutants to the generation of renewable solar fuels, yet our understanding of the mechanisms has been hindered by the multistep nature of the process and the wide range of time scales over which it occurs (femtoseconds to seconds). In this work, we use ultrafast laser pump-probe techniques to follow the time evolution of substrate-induced photooxidation of acetone on a titania surface. A UV light at 260 nm initiates carrier-induced fragmentation of adsorbed acetone on a TiO2(110) surface that was pretreated with oxygen. The photoreaction results in the ejection of methyl radicals into the gas-phase that are detected by the probe pulse via resonant multiphoton ionization. The time evolution of the methyl radicals leaving the surface exhibits ultrafast rise times, 300-700 fs, followed by a more gradual rise that plateaus by 10 ps, with faster rates at a low acetone coverage. These results are interpreted in terms of a time-dependent rate expression and a mechanism in which the fragmentation of the acetone surface species is driven by interactions with nonequilibrium, "hot" holes.

Branching Ratios of the N(2D03/2) and N(2D05/2) Spin-Orbit States Produced in the State-Selected Photodissociation of N2 Determined Using Time-Sliced Velocity-Mapped-Imaging Photoionization Mass Spectrometry (TS-VMI-PI-MS).

Branching ratios for N(2D03/2) and N(2D05/2) produced by predissociation of state selected excited nitrogen molecules in the vacuum ultraviolet region have been measured for the first time. The quantum numbers of the excited nitrogen molecule are defined by selective excitation of the nitrogen molecule in the Franck-Condon region from the ground electronic, 1Σg+, vibrational, v″, and rotational, J″ state to an excited Eu', v', J' state with a tunable vacuum ultraviolet, VUV1, laser. The neutral atoms produced by predissociation from this excited state are then selectively ionized with a second tunable VUV2 laser. Measurement of the relative populations of these two atoms formed in their spin-orbit states defines the quantum states for the atomic products. This means that the wave functions of the initial state and knowledge of the relative yields define all the experimental parameters for this series of unimolecular reactions. The ions formed by VUV2 are mass analyzed with a time-of-flight mass spectrometer and detected with a time slice velocity ion imaging mass spectrometer. In this manner, we can determine the recoil velocity associated with the predissociation process. Two different techniques are used to determine the spin-orbit ratios, namely, resonant VUV photoionization (RVUV-PI) spectroscopy and total kinetic energy release (TKER) spectroscopy determined from the image produced when the atoms are selectively ionized by VUV2 in the interaction region. The TKER spectra obtained from the lines at 110 296.25 and 110 304.96 cm-1 that couple to a newly discovered autoionization line at 129 529.4255 ± 0.0015 cm-1 prove that the lines observed in this region originate from the N(2D03/2) and N(2D05/2) atoms. Two other lines in this region at 110 286.20 and 110 299.89 cm-1 originate from the nitrogen N(4S03/2) that is photoionized in a 1+ 1 VUV-UV resonant multiphoton ionization process. The spin-orbit branching ratios have been evaluated for valence and Rydberg electronic excited states from 104 129.4 to 118 772.1 cm-1, and it shows that they are independent of the rotational and vibrational quantum numbers. They are not appreciably affected by the symmetry properties of the wave function in the Franck-Condon region of the excited states. In the energy region below 117 153.8 cm-1 the pathways at long internuclear distances appear to determine [N(2D03/2)]/[N(2D05/2)] branching ratios of ∼0.38, ∼0.62, and ∼1.04. At higher energies, TKER and RVUV-PI spectroscopy have been used to show that the average fraction of the N(2D03/2) and N(2D05/2) atoms produced in the spin-allowed channels that produce two N(2D0J) is 0.85 versus 0.15 for spin-forbidden channels. The importance and need for this information for comparison with theory and applications in astrochemistry are briefly discussed.

Electron‐Nuclear Dynamics in Molecular Harmonic Generation Driven by a Plasmonic Nonhomogeneous Field

Electron (z)‐nuclear (R) dynamics in the molecular high‐order harmonic generation (MHHG) from H2+ driven by the plasmonic nonhomogeneous field, generated by the surface plasmon polaritons in the bowtie‐shaped nanostructure, have been theoretically investigated through solving the two dimensional time‐dependent Schrödinger equation with the Non‐Bohn‐Oppenheimer approximation. It is found that (i) due to the plasmonic enhancement of the laser intensity, the harmonic cutoff can be extended when the spatial position of H2+ is away from the gap center of the nanostructure. However, due to the limit of the gap size, the threshold value of the harmonic cutoff can be obtained at a given position of H2+. (ii) Due to the asymmetric enhancement of the laser intensity in space, the extended higher harmonics are respectively from E(t) > 0 a.u. or E(t) < 0 a.u. for the cases of the positive and the negative spatial position of H2+. As a result, the intensities of the extended higher harmonics are different and can be controlled by changing the carrier‐envelope phase and the pulse duration of the laser field. (iii) In the few‐cycle pulse duration, the MHHG mainly comes from the multi‐photon resonance ionization (MPRI), while as the pulse duration increases, the MPRI, the charge‐resonance enhanced ionization (CREI) and even the dissociative ionization (DI) are contributed to the MHHG. Moreover, as the spatial position of H2+ moves, the contributions of the MHHG from the MPRI, the CERI and the DI can be controlled. (iv) The contributions of the MHHG from the two‐H nuclei have been investigated and found that when E(t) > 0 a.u., the intensities of the harmonics from the negative‐H is higher than those from the positive‐H; while when E(t) < 0 a.u., the intensities of the harmonics from the positive‐H plays the main role in the MHHG. Moreover, the multi‐minima, caused by the two‐center interference can also be found. (v) Finally, by superposing a properly selected harmonics, a single isolated attosecond pulse (SIAP) with the full width at half maximum (FWHM) of 34 as can be obtained.

Molecular harmonic generation in bowtie-shaped nanostructure

Molecular high-order harmonic generation (MHHG) from in the bowtie-shaped nanostructure has been investigated. The results show that (i) due to the surface plasmon polaritons in the bowtie-shaped nanostructure, the laser intensity can be enhanced and present the nonhomogeneous effect in space. As a result, the extension of the harmonic cutoff can be achieved when the position of is injected away from the gap center of the nanostructure. However, due to the limited gap size of the nanostructure, the inherent threshold value of the harmonic cutoff can be found. (ii) Due to the asymmetric enhancement of the laser intensity in space, the extended higher harmonics come from E(t) > 0 or E(t) < 0 for the cases of the positive or the negative injected positions of , respectively. (iii) In the few-cycle pulse, the extended higher harmonics mainly comes from the multi-photon resonance ionization (MPRI) around R = 4.0 and R = 4.5 but from only one harmonic emission event (HEE), thus leading to the intensity difference of the extended higher harmonics from the positive and the negative positions of . In the multi-cycle pulse, the extended higher harmonics are also attributed to the MPRI but from R = 3.5 to R = 6.0 and there are three HEEs contributed to the extended higher harmonics, thus leading to the similar intensity of the extended higher harmonics from the positive and the negative positions of . (iv) Due to the slower nuclear motion of the heavy nuclei, the intensities of the harmonics from are lower than those from . (v) The contributions of the MHHG from the two-H nuclei show that when E(t) > 0 or E(t) < 0, the left-H or the right-H plays the dominating role in the MHHG. Moreover, as moves away from the gap center, the intensity difference of the higher harmonics from the two-H nuclei can be enhanced. But, it can be decreased as the pulse duration increases.

Chirp control of multi-photon resonance ionization and charge-resonance enhanced ionization on molecular harmonic generation

The effects of the multi-photon resonance ionization (MPRI) and the charge-resonance enhanced ionization (CREI) on the molecular high-order harmonic generation (MHHG) from H2+ have been investigated by using the chirped pulses. It is found that the MHHG only comes from the MPRI in the shorter pulse duration. As the pulse duration increases, both the MPRI and the CREI contribute to the MHHG. But the MPRI plays the main role in the generations of the above-threshold harmonics and the CREI mainly contributes to the below-threshold harmonics. With the introductions of the up-chirped and the down-chirped pulses, the contributions of the MHHG from the CREI and the MPRI can be enhanced, respectively. Finally, the isotopic investigation (e.g. T2+) shows that due to the slower nuclear motion of the heavy nuclei, the contributions of MHHG from the CERI can be suppressed in the heavy nuclei.

Influence of Doppler effect on the absorption in the process of resonance multiphoton ionization

The theory of density matrix equation is used to describe the interaction between laser field and atoms or molecules (referred as particles). With this theory, this paper investigated the influence of Doppler effect on the resonance absorption in the process of resonance enhanced multiphoton ionization (REMPI) of particles. The results show that at the beginning of laser radiation, Doppler effect will broaden the absorption line. This is generally known as Doppler broadening. After laser radiates for a period of time, light transparency will appear at the position of near zero resonance detuning. This is caused by AC Stark splitting. Thus this phenomenon appears only when Rabi frequency is large enough. Doppler effect can make light transparency disappear, because the gap between the two peaks beside of transparent window is smaller than the line-width of Doppler broadening. It will also enlarge the Rabi frequency for transparency.

Resonant multiphoton ionisation probe of the photodissociation dynamics of ammonia.

The dissociation dynamics of the Ã-state of ammonia have been studied using a resonant multiphoton ionisation probe in a photoelectron spectroscopy experiment. The use of a resonant intermediate in the multiphoton ionisation process changes the ionisation propensity, allowing access to different ion states when compared with equivalent single photon ionisation experiments. Ionisation through the E' 1A1' Rydberg intermediate means we maintain overlap with the ion state for an extended period, allowing us to monitor the excited state population for several hundred femtoseconds. The vibrational states in the photoelectron spectrum show two distinct timescales, 200 fs and 320 fs, that we assign to the non-adiabatic and adiabatic dissociation processes respectively. The different timescales derive from differences in the wavepacket trajectories for the two dissociation pathways that resonantly excite different vibrational states in the intermediate Rydberg state. The timescales are similar to those obtained from time resolved ion kinetic energy release measurements, suggesting we can measure the different trajectories taken out to the region of conical intersection.

Investigation on absorptive and dispersive properties in the process of resonance multiphoton ionization

Abstract The absorptive and dispersive properties in the process of resonance enhanced multiphoton ionization are investigated theoretically. It is shown that resonance absorption presents the character of transparency at the position of near resonance frequency, and a weak anomalous dispersion appears at the same time. Both transparent depth and the dispersion increase with the reduction of ionization ability. The phenomenon of transparency and weakened dispersion will emerge also when Rabi frequency is larger than 2 GHz, but the dispersive phenomenon will disappear if Rabi frequency is large enough. Thus we can realize complex light propagate with the same speed.

Selective enhancement of resonant multiphoton ionization with strong laser fields

High-resolution photoelectron momentum distributions of Xe atoms ionized by 800-nm linearly polarized laser fields have been traced at intensities from 1.1*1013 to 3.5*1013W/cm2 using velocity-map imaging techniques. At certain laser intensities, the momentum spectrum exhibits a distinct double-ring structure for low-order above-threshold ionization, which appears to be absent at lower or higher laser intensities. By investigating the intensity-resolved photoelectron energy spectrum, we find that this double-ring structure originates from resonant multiphoton ionization involving multiple Rydberg states of atoms. Varying the laser intensity, we can selectively enhance the resonant multiphoton ionization through certain atomic Rydberg states. The photoelectron angular distributions of multiphoton resonance are also investigated for the low-order above-threshold ionization.

Gas phase double-resonance IR/UV spectroscopy of an alanine dipeptide analogue using a non-covalently bound UV-tag: observation of a folded peptide conformation in the Ac-Ala-NH2–toluene complex

The conformation-selective IR spectroscopy of the alanine dipeptide analogue Ac-Ala-NH2, a molecule deprived of a UV chromophore, has been obtained by laser double-resonance IR/UV spectroscopy, using a UV-tag (toluene) non-covalently bound to the peptide. The Ac-Ala-NH2–toluene complex, as detected through a resonant multiphoton ionisation process, is composed of a folded peptide monomer (C7 conformation) attached to the toluene molecule through an NH–π H-bond. Quantum chemistry calculations suggest that the structure of this complexed folded peptide is very similar to that of the isolated peptide, legitimating this experimental strategy for investigating the peptide structure. The population of exclusively folded peptides, in contrast to the C5/C7 conformational mixture observed on isolated monomers measured with other techniques, suggest that conformational changes might not only be induced by complexation of a single polar solvent molecule as previously reported, but also by an apolar, weakly interacting molecule like toluene.

Oriented O(3P2), Ne(3P2), and He(3S1) atoms emerging from a bent magnetic guide

ABSTRACTWe describe our observation of strongly oriented total electronic angular momentum J in O(3P2), Ne(3P2), and He(3S1) atoms emerging from a bent magnetic multipole guide, as measured by resonant multiphoton ionisation. This was contrary to our expectation because no additional (uniform) magnetic fields were applied to orient the atoms behind the exit of the guide. Two- and three-photon ionisation techniques were employed to determine the degree of J polarisation, from which we infer that atoms become oriented as a result of a combination of weak fringe fields, possible stray fields, and the fact that molecular beam packets do not oscillate around the geometric center of the bent multipole guide. We conclude that similar effects may exist in other, related experiments and that a detailed characterisation of the degree of orientation is required prior to any study of chemical dynamics or spectroscopy. This paper should serve as a warning for anybody using similar devices not to assume isotropic angular momentum distributions of atoms and molecules emerging from a magnetic guide or a decelerator, particularly when it is bent; whenever possible, the possibility for a J anisotropy should be experimentally checked.

Molecular photoelectron angular distribution rotations in multi-photon resonant ionization of H2(+) by circularly polarized ultraviolet laser pulses.

We study effects of pulse durations on molecular photoelectron angular distributions (MPADs) in ultrafast circular polarization ultraviolet resonant ionization processes. Simulations performed on aligned H2 (+) by numerically solving time dependent Schrödinger equations show rotations of MPADs with respect to the molecular symmetry axes. It is found that in multi-photon resonant ionization processes, rotation angles are sensitive to pulse durations, which we attribute to the coherent resonant excitation between the ground state and the intermediate excited electronic state induced by Rabi oscillations. Multi-photon nonresonant and single photon ionization processes are simulated and compared which exhibit a constant rotation angle. An asymmetry parameter is introduced to describe the pulse duration sensitivity by perturbation theory models. Influence of pulse frequency detunings on MPADs is also investigated where oscillations of rotations are absent at long pulse durations due to nonresonance excitation.

Resonant and nonresonant multiphoton ionization processes in the mass spectrometry of explosives.

Multiphoton ionization processes were studied for three types of explosives using a line-tunable ultraviolet femtosecond laser. When peroxides such as triacetone triperoxide (TATP) and hexamethylene triperoxide diamine (HMTD) were ionized through a nonresonant two-photon process, a molecular ion was dominantly observed by reducing the excess energy remaining in the ion. However, an aromatic nitro compound such as 2,4,6-trinitrotoluene (TNT) produced large signals arising from molecular and fragment ions by resonant two-photon ionization. In addition, only fragment ions were produced from a nonaromatic nitro compound such as 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), even when a resonant two-photon ionization process was employed, suggesting that a further reduction in excess energy would be necessary if a molecular ion were to be observed.

Self-interaction corrected density functional calculations of Rydberg states of molecular clusters: N,N-dimethylisopropylamine.

Theoretical calculations of Rydberg excited states of molecular clusters consisting of N,N-dimethylisopropylamine molecules using a Perdew-Zunger self-interaction corrected energy functional are presented and compared with results of resonant multiphoton ionization measurements. The binding energy of the Rydberg electron in the monomer is calculated to be 2.79 eV and 2.27 eV in the 3s and 3p state, respectively, which compares well with measured values of 2.88 eV and 2.21 eV. Three different stable configurations of the dimer in the ground state were found using an energy functional that includes van der Waals interaction. The lowest ground state energy conformation has the two N-atoms widely separated, by 6.2 Å, while the Rydberg state energy is lowest for a configuration where the N-atoms of the two molecules come close together, separated by 3.7 Å. This conformational change is found to lower the Rydberg electron binding energy by 0.2 eV. The self-interaction corrected functional gives a highly localized hole on one of the two molecules, unlike results obtained using the PBE functional or the hybrid B3LYP functional which give a delocalized hole. For the trimer, the self-interaction corrected calculation gives a Rydberg electron binding energy lowered further by 0.13 eV as compared with the dimer. The calculated results compare well with trends observed in experimental measurements. The reduction of the Rydberg electron binding energy with cluster size can be ascribed to an effective delocalization of the positive charge of the hole by the induced and permanent dipole moments of the neighboring molecules. A further decrease observed to occur on a time scale of tens of ps can be ascribed to a structural rearrangement of the clusters in the Rydberg state where molecules rotate to orient their dipoles in response to the formation of the localized hole.