Investigation on the vibrational relaxation and ultrafast electronic dynamics of S1 state in 2,4-difluoroanisole

Intramolecular vibrational energy redistribution (IVR) has a profound impact on dynamic processes. We have studied two types of IVR processes, restricted and dissipative, and ultrafast dynamics of the S1 state of 2,4-difluoroanisole using time-resolved photoelectron spectroscopy and time-of-flight mass spectroscopy. The restricted IVR occurs in the intermediate regime of 219 cm-1 vibrational level, and the dissipative IVR occurs in the statistical regime of 1200 cm-1. The lifetimes of IVR processes are measured to be 90 and 11 ps, respectively, depending on the internal energies of the S1 state and differ by a factor of eight. Similar subsequent dynamics were observed at two vibrational levels in the S1 state. The population undergoes IVR following the initial excitation and subsequently leaks into a triplet state, accompanied by intersystem crossing within ∼400 ps followed by a slower nonradiative relaxation of the triplet state on the nanosecond time scale. Furthermore, the values of 3s and 3px Rydberg states of 2,4-difluoroanisole were experimentally determined to be 5.02 and 6.28 eV.

Strong Field Ionization-Photofragmentation on Ultrafast Evolution of Electronic States of Toluene Cations

Ultrafast time-resolved strong field ionization-photofragmentation (SFI-PF) has emerged as a useful method for investigation of dynamics of molecular cations. Here we perform a SFI-PF study on the electronic states of toluene cations. By measuring the ion yields as a function of delay time, we obtain the transients of both parent and daughter ions, which show ultrafast decays and out-of-phase oscillations. The results provide the first experimental evidence of D₁-D₀ ultrafast relaxation of toluene cations occurring in about 530 fs and indicate coincident resonance between the vibrational states in D₁ and D₀ leading to oscillations with a period of about 2.05 ps. Our study should shed some light on the ultrafast photochemistry involving complex molecular cations.

Real-Time Observation of Fermi Resonances in the S1 State of Phenol

Fermi resonances in the first electronically excited (S₁) state of phenol have been observed in real time. Quantum beats associated with coherent superposition of Fermi resonant eigenstates are manifested as temporal oscillations of the ionization cross sections of which the amplitudes are strongly dependent on the total ionization energy. This indicates that coherently excited eigenstates are effectively decomposed into their zeroth-order states, providing the unique opportunity for the investigation of nonstationary state dynamics in real-time. Energy gaps (Δν̃) of eigenstates within the laser coherence width have been most precisely determined up to date, giving Δν̃ ∼ 3.302 ± 0.001 or 1.655 ± 0.001 cm^-1 for the 1¹/4¹10b¹ or 12²/8a¹ Fermi doublets, respectively. Dephasing rate suddenly increases as the S₁ internal energy becomes above ∼1500 cm^-1, revealing the important role of energy randomization dynamics during the H atom tunneling process of phenol in S₁.

Effects of symmetry, methyl groups and serendipity on intramolecular vibrational energy dispersal

Intramolecular vibrational dispersal of vibrational energy is more efficient in the symmetrically-substituted p-xylene molecule than in p-fluorotoluene, p-chlorofluorobenzene or p-difluorobenzene.

Wavepacket insights into the photoprotection mechanism of the UV filter methyl anthranilate

Meradimate is a broad-spectrum ultraviolet absorber used as a chemical filter in commercial sunscreens. Herein, we explore the ultrafast photodynamics occurring in methyl anthranilate (precursor to Meradimate) immediately after photoexcitation with ultraviolet radiation to understand the mechanisms underpinning Meradimate photoprotection. Using time-resolved photoelectron spectroscopy, signal from the first singlet excited state of methyl anthranilate shows an oscillatory behavior, i.e., quantum beats. Our studies reveal a dependence of the observed beating frequencies on photoexcitation wavelength and photoelectron kinetic energy, unveiling the different Franck-Condon overlaps between the vibrational levels of the ground electronic, first electronic excited, and ground cationic states of methyl anthranilate. By evaluating the behavior of these beats with increasing photon energy, we find evidence for intramolecular vibrational energy redistribution on the first electronic excited state. Such energy redistribution hinders efficient relaxation of the electronic excited state, making methyl anthranilate a poor choice for an efficient, efficacious sunscreen chemical filter.

Here, the authors explore the ultrafast photodynamics of methyl anthranilate. From the quantum beat behavior, the authors find evidence for ultrafast energy redistribution processes which hinder excited state relaxation, making methyl anthranilate a poor choice for a sunscreen chemical filter.

Direct observation of vibrational energy dispersal via methyl torsions

Explicit evidence for the role of methyl rotor levels in promoting energy dispersal is reported. A set of coupled zero-order vibration/vibration-torsion (vibtor) levels in the S₁ state of para-fluorotoluene (pFT) are investigated. Two-dimensional laser-induced fluorescence (2D-LIF) and two-dimensional zero-kinetic-energy (2D-ZEKE) spectra are reported, and the assignment of the main features in both sets of spectra reveals that the methyl torsion is instrumental in providing a route for coupling between vibrational levels of different symmetry classes. We find that there is very localized, and selective, dissipation of energy via doorway states, and that, in addition to an increase in the density of states, a critical role of the methyl group is a relaxation of symmetry constraints compared to direct vibrational coupling.

Femtosecond time-resolved observation of butterfly vibration in electronically excited o-fluorophenol

The butterfly vibration during the hydrogen tunneling process in electronically excited o-fluorophenol has been visualized in real time by femtosecond time-resolved ion yield spectroscopy coupled with time-resolved photoelectron imaging technique. A coherent superposition of out-of-plane C–F butterfly motions is prepared in the first excited electronic state (S₁). As the C–F bond vibrates with respect to the aromatic ring, the nuclear geometry varies periodically, leading to the corresponding variation in the photoionization channel. By virtue of the more favorable ionization probability from the nonplanar minimum via resonance with the Rydberg states, the evolution of the vibrational wave packet is manifested as a superimposed beat in the parent-ion transient. Moreover, time-resolved photoelectron spectra offer a direct mapping of the oscillating butterfly vibration between the planar geometry and nonplanar minimum. The beats for the photoelectron peaks originating from the planar geometry are out of phase with those from the nonplanar minimum. Our results provide a physically intuitive and complete picture of the oscillatory flow of energy responsible for the coherent vibrational motion on the excited state surface.

Vibrational and vibrational-torsional interactions in the 0-600 cm-1 region of the S1← S0 spectrum of p-xylene investigated with resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy

We assign the 0-600 cm^-1 region of the S₁← S₀ transition in p-xylene (p-dimethylbenzene) using resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy. In the 0-350 cm^-1 range as well as the intense origin band, there are a number of torsional and vibration-torsion (vibtor) features. The latter are discussed in more detail in Paper I [A. M. Gardner et al., J. Chem. Phys. 146, 124308 (2017)]. Here we focus on the origin and the 300-600 cm^-1 region, where vibrational bands and some vibtor activity are observed. From the origin ZEKE spectrum, we derive the ionization energy of p-xylene as 68200 ± 5 cm^-1. The assignment of the REMPI spectrum is based on the activity observed in the ZEKE spectra coupled with knowledge of the vibrational wavenumbers obtained from quantum chemical calculations. We assign several isolated vibrations and a complex Fermi resonance that is found to comprise contributions from both vibrations and vibtor levels, and we examine this via a two-dimensional ZEKE spectrum. A number of the vibrational features in the REMPI and ZEKE spectra of p-xylene that have been reported previously are reassigned and now largely consist of totally symmetric contributions. We briefly discuss the appearance of non-Franck-Condon allowed transitions. Finally, we find remarkably similar spectral activity to that in the related disubstituted benzenes, para-difluorobenzene, and para-fluorotoluene.

Probing the origins of vibrational mode specificity in intramolecular dynamics through picosecond time-resolved photoelectron imaging studies

Enhanced torsion-vibration coupling associated with a selected vibrational mode is shown to accelerate intramolecular energy flow in p-fluorotoluene.

Torsion and vibration-torsion levels of the S1 and ground cation electronic states of para-fluorotoluene

We investigate the low-energy transitions (0-570 cm^-1) of the S₁ state of para-fluorotoluene (pFT) using a combination of resonance-enhanced multiphoton ionization and zero-kinetic-energy (ZEKE) spectroscopy and quantum chemical calculations. By using various S₁ states as intermediate levels, we obtain ZEKE spectra. The differing activity observed allows detailed assignments to be made of both the cation and S₁ low-energy levels. The assignments are in line with the recently published work on toluene from the Lawrance group [J. R. Gascooke et al., J. Chem. Phys. 143, 044313 (2015)], which considered vibration-torsion coupling in depth for the S₁ state of toluene. In addition, we investigate whether two bands that occur in the range 390-420 cm^-1 are the result of a Fermi resonance; we present evidence for weak coupling between various vibrations and torsions that contribute to this region. This work has led to the identification of a number of misassignments in the literature, and these are corrected.

Complex and Sustained Quantum Beating Patterns in a Classic IVR System: The 3(1)5(1) Level in S1 p-Difluorobenzene

Using picosecond time-resolved photoelectron imaging, we have studied the intramolecular vibrational energy redistribution (IVR) dynamics that occur following the excitation of the 3(1)5(1) level, which lies 2068 cm(-1) above the S1 origin in p-difluorobenzene. Our technique, which has superior time resolution to that of earlier studies but retains sufficient energy resolution to identify the behavior of individual vibrational states, enables us to determine six distinct beating periods in photoelectron intensity, only one of which has been observed previously. Analysis shows that the IVR dynamics are restricted among only a handful of vibrational levels, despite the relatively high excitation energy. This is deduced to be a consequence of the high symmetry and rigid structure of p-difluorobenzene.

Vibrations of the low energy states of toluene (X̃ (1)A1 and à (1)B2) and the toluene cation (X̃ (2)B1)

We commence by presenting an overview of the assignment of the vibrational frequencies of the toluene molecule in its ground (S0) state. The assignment given is in terms of a recently proposed nomenclature, which allows the ring-localized vibrations to be compared straightforwardly across different monosubstituted benzenes. The frequencies and assignments are based not only on a range of previous work, but also on calculated wavenumbers for both the fully hydrogenated (toluene-h8) and the deuterated-methyl group isotopologue (α3-toluene-d3), obtained from density functional theory (DFT), including artificial-isotope shifts. For the S1 state, one-colour resonance-enhanced multiphoton ionization (REMPI) spectroscopy was employed, with the vibrational assignments also being based on previous work and time-dependent density functional theory (TDDFT) calculated values; but also making use of the activity observed in two-colour zero kinetic energy (ZEKE) spectroscopy. The ZEKE experiments were carried out employing a (1 + 1(')) ionization scheme, using various vibrational levels of the S1 state with an energy <630 cm(-1) as intermediates; as such we only discuss in detail the assignment of the REMPI spectra at wavenumbers <700 cm(-1), referring to the assignment of the ZEKE spectra concurrently. Comparison of the ZEKE spectra for the two toluene isotopologues, as well as with previously reported dispersed-fluorescence spectra, and with the results of DFT calculations, provide insight both into the assignment of the vibrations in the S1 and D0(+) states, as well as the couplings between these vibrations. In particular, insight into the nature of a complicated Fermi resonance feature at ∼460 cm(-1) in the S1 state is obtained, and Fermi resonances in the cation are identified. Finally, we compare activity observed in both REMPI and ZEKE spectroscopy for both toluene isotopologues with that for fluorobenzene and chlorobenzene.

Elucidating quantum number-dependent coupling matrix elements using picosecond time-resolved photoelectron spectroscopy

We measure quantum beating patterns of photoelectron intensity caused by intramolecular vibrational energy redistribution following the excitation of a low-lying ring breathing state in S(1) parafluorotoluene. Analysis of the beating patterns reveals an exceptional sensitivity to details of the evolving wave packet which is found to contain two incoherent components, one of which rapidly dephases. This analysis enables the determination of coupling matrix elements, which are shown to depend strongly on torsional and rotational quantum numbers.

Intramolecular vibrational dynamics in S1 p-fluorotoluene. I. Direct observation of doorway states

Picosecond time-resolved photoelectron spectroscopy is used to investigate intramolecular vibrational redistribution (IVR) following excitation of S(1) 18a(1) in p-fluorotoluene (pFT) at an internal energy of 845 cm(-1), where ν(18a) is a ring bending vibrational mode. Characteristic oscillations with periods of 8 ps and 5 ps are observed in the photoelectron signal and attributed to coupling between the initially excited zero-order bright state and two doorway states. Values for the coupling coefficients connecting these three vibrational states have been determined. In addition, an exponential change in photoelectron signal with a lifetime of 17 ps is attributed to weaker couplings with a bath of dark states that play a more significant role during the latter stages of IVR. A tier model has been used to assign the most strongly coupled doorway state to S(1) 17a(1) 6a(2)('), where ν(17a) is a CH out-of-plane vibrational mode and 6a(2)(') is a methyl torsional level. This assignment signifies that a torsion-vibration coupling mechanism mediates the observed dynamics, thus demonstrating the important role played by the methyl torsional mode in accelerating IVR.

Time-resolved photoelectron spectroscopy: from wavepackets to observables

Time-resolved photoelectron spectroscopy (TRPES) is a powerful tool for the study of intramolecular dynamics, particularly excited state non-adiabatic dynamics in polyatomic molecules. Depending on the problem at hand, different levels of TRPES measurements can be performed: time-resolved photoelectron yield; time- and energy-resolved photoelectron yield; time-, energy-, and angle-resolved photoelectron yield. In this pedagogical overview, a conceptual framework for time-resolved photoionization measurements is presented, together with discussion of relevant theory for the different aspects of TRPES. Simple models are used to illustrate the theory, and key concepts are further amplified by experimental examples. These examples are chosen to show the application of TRPES to the investigation of a range of problems in the excited state dynamics of molecules: from the simplest vibrational wavepacket on a single potential energy surface; to disentangling intrinsically coupled electronic and nuclear motions; to identifying the electronic character of the intermediate states involved in non-adiabatic dynamics by angle-resolved measurements in the molecular frame, the most complete measurement.