Mode-specific excited-state dynamics of N-methylpyrrole

Comment on "Electronic, vibrational and torsional couplings in N-methylpyrrole: Ground, first excited and cation states" [J. Chem. Phys. 154, 224305 (2021)]

Two-color (1 + 1') zero-electron-kinetic-energy (ZEKE) and photoionization efficiency (PIE) spectra are reported via different levels in the S₁ ← S₀ (ùA₂←X̃¹A₁) one-photon transition of jet-cooled N-methylpyrrole. The laser radiation is produced using two dye lasers, one with an 1800 l/mm grating and one with 2400 l/mm. We report spectra where the excitation and ionization radiation are produced with both combinations of the dye lasers; these spectra differ markedly. This is attributed to Wood's anomalies with the 2400 l/mm grating: one aspect is a loss in light intensity over a range of wavelengths, attributed to a resonance anomaly. Another is the appearance of a "shadow" ZEKE spectrum and PIE curve at apparently higher ionization wavenumbers; under some conditions, a third ZEKE spectrum was observed-these latter observations arise from higher-order dispersion effects, likely caused by a Rayleigh anomaly. We comment on these observations and report more representative ZEKE and PIE spectra than those presented in a recent paper by our group [A. R. Davies, D. J. Kemp, and T. G. Wright, J. Chem. Phys. 154, 224305 (2021)] for four intermediate S₁ levels.

Ultrafast decay dynamics of electronically excited 2-ethylpyrrole

The excited-state decay dynamics of 2-ethylpyrrole following UV excitation in the wavelength range of 254.8-218.0 nm is investigated in detail using the femtosecond time-resolved photoelectron imaging method. The time-resolved photoelectron spectra and photoelectron angular distributions at all pump wavelengths are carefully analysed and the following picture is derived: at the longest pump wavelengths (254.8, 248.3 and 246.1 nm), 2-ethylpyrrole is excited to the S₁(¹πσ*) state having a lifetime of about 50 fs. At 248.3, 246.1 and 237.4 nm, another excited state of Rydberg character is excited. The lifetime of this state is ∼570 fs at 237.4 nm and becomes slightly longer at other two pump wavelengths. At the shortest pump wavelengths (230.8 and 218.0 nm), 2-ethylpyrrole is excited to a state which is tentatively assigned to the 1¹ππ* state, having a lifetime of 75 ± 15 and 48 ± 10 fs for the longer and shorter pump wavelengths, respectively. Internal conversion to the S₁(¹πσ*) state might be one of the decay mechanisms of the 1¹ππ* state.

Electronic, vibrational, and torsional couplings in N-methylpyrrole: Ground, first excited, and cation states

The electronic spectrum associated with the S₁ ← S₀ (ùA₂←X̃¹A₁) one-photon transition of jet-cooled N-methylpyrrole is investigated using laser-induced fluorescence (LIF) and (1 + 1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy; in addition, the (2 + 2) REMPI spectrum is considered. Assignment of the observed bands is achieved using a combination of dispersed fluorescence (DF), two-dimensional LIF (2D-LIF), zero-electron-kinetic energy (ZEKE) spectroscopy, and quantum chemical calculations. The spectroscopic studies project the levels of the S₁ state onto those of either the S₀ state, in DF and 2D-LIF spectroscopy, or the ground state cation (D₀ ⁺) state, in ZEKE spectroscopy. The assignments of the spectra provide information on the vibrational, vibration-torsion (vibtor), and torsional levels in those states and those of the S₁ levels. The spectra are indicative of vibronic (including torsional) interactions between the S₁ state and other excited electronic states, deduced both in terms of the vibrational activity observed and shifts from expected vibrational wavenumbers in the S₁ state, attributed to the resulting altered shape of the S₁ surface. Many of the ZEKE spectra are consistent with the largely Rydberg nature of the S₁ state near the Franck-Condon region; however, there is also some activity that is less straightforward to explain. Comments are made regarding the photodynamics of the S₁ state.

Conformer-Specific Tunneling Dynamics Dictated by the Seam Coordinate of the Conical Intersection

The dynamic role of the conical intersection "seam" coordinate has been first revealed in the H fragmentation reaction of ortho(o)-cresol conformers. One of the (3N - 8) dimensional seam coordinates of the S₁(ππ*)/S₂(πσ*) conical intersection has been identified as the CH₃ torsional potential function. The tunneling dynamics of the reactive flux is dictated by its nuclear layout with respect to the CH₃ torsional angle, as the multidimensional tunneling barrier is dynamically shaped along the conical intersection seam. The effective tunneling-barrier weight-averaged over the quantum-mechanical probability along the CH₃ torsional angle perfectly explains the experimental finding: the sharp variation of the tunneling rate ((700-400) ps^-1) with the CH₃ torsional mode excitations within the narrow (0-100 cm^-1) energetic window. The much longer S₁ lifetime of cis compared to trans is ascribed to the higher-lying S₁/S₂ conical intersection of the former. With the use of distinct lifetimes, vibronic bands of each conformer could be completely separated.

Real-Time Tunneling Dynamics through Adiabatic Potential Energy Surfaces Shaped by a Conical Intersection

Dynamic shaping of the adiabatic tunneling barrier in the S-H bond extension coordinate of several ortho-substituted thiophenols has been found to be mediated by low-frequency out-of-plane vibrational modes, which are parallel to the coupling vector of the branching plane comprising the conical intersection. The S-H predissociation tunneling rate (k) measured when exciting to the S₁ zero-point level of 2-methoxythiophenol (44 ps)^-1 increases abruptly, to k ≈ (22 ps)^-1, at the energy corresponding to excitation of the 152 cm^-1 out-of-plane vibrational mode and then falls back to k ≈ (40 ps)^-1 when the in-plane mode is excited at 282 cm^-1. Similar resonance-like peaks in plots of S₁ tunneling rate versus internal energy are observed when exciting the corresponding low-frequency out-of-plane modes in the S₁ states of 2-fluorothiophenol and 2-chlorothiophenol. This experiment provides clear-cut evidence for dynamical "shaping" of the lower-lying adiabatic potential energy surfaces by the higher-lying conical intersection seam, which dictates the multidimensional tunneling dynamics.

Vibronic structure and predissociation dynamics of 2-methoxythiophenol (S1): The effect of intramolecular hydrogen bonding on nonadiabatic dynamics

Vibronic spectroscopy and the S-H bond predissociation dynamics of 2-methoxythiophenol (2-MTP) in the S₁ (ππ^*) state have been investigated for the first time. Resonant two-photon ionization and slow-electron velocity map imaging (SEVI) spectroscopies have revealed that the S₁-S₀ transition of 2-MTP is accompanied with the planar to the pseudoplanar structural change along the out-of-plane ring distortion and the tilt of the methoxy moiety. The S₁ vibronic bands up to their internal energy of ∼1000 cm^-1 are assigned from the SEVI spectra taken via various S₁ vibronic intermediate states with the aid of ab initio calculations. Intriguingly, Fermi resonances have been identified for some vibronic bands. The S-H bond breakage of 2-MTP occurs via tunneling through an adiabatic barrier under the S₁/S₂ conical intersection seam, and it is followed by the bifurcation into either the adiabatic or nonadiabatic channel at the S₀/S₂ conical intersection where the diabatic S₂ state (πσ^*) is unbound with respect to the S-H bond elongation coordinate, giving the excited (Ã) or ground (X̃) state of the 2-methoxythiophenoxy radical, respectively. Surprisingly, the nonadiabatic transition probability at the S₀/S₂ conical intersection, estimated from the velocity map ion images of the nascent D fragment from 2-MTP-d₁ (2-CH₃O-C₆H₄SD) at the S₁ zero-point energy level, is found to be exceptionally high to give the X̃/Ã product branching ratio of 2.03 ± 0.20, which is much higher than the value of ∼0.8 estimated for the bare thiophenol at the S₁ origin. It even increases to 2.33 ± 0.17 at the ν₄₅ ² mode (101 cm^-1) before it rapidly decays to 0.69 ± 0.05 at the S₁ internal energy of about 2200 cm^-1. This suggests that the strong intramolecular hydrogen bonding of S⋯D⋯OCH₃ in 2-MTP at least in the low S₁ internal energy region should play a significant role in localizing the reactive flux onto the conical intersection seam. The minimum energy pathway calculations (second-order coupled-cluster resolution of the identity or time-dependent-density functional theory) of the adiabatic S₁ state suggest that the intimate dynamic interplay between the S-H bond cleavage and intramolecular hydrogen bonding could be crucial in the nonadiabatic surface hopping dynamics taking place at the conical intersection.