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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. [ABSTRACT FROM AUTHOR]

The non-adiabatic relaxation processes and the fragmentation dynamics of Rydberg-excited N,N,N′,N′-tetramethylmethylenediamine (TMMDA) are investigated using femtosecond time-resolved photoelectron imaging and time-resolved mass spectroscopy. Excitation at 208 nm populates TMMDA in a charge-localized 3p state. Rapid internal conversion (IC) to 3s produces two charge-delocalized conformers with independent time constants and distinct population ratios. As the system explores the 3s potential surface, the structural evolution continues on a 1.55 ps timescale, followed by a slower (12.1 ps) relaxation to the ground state. A thorough comparison of the time-dependent mass and photoelectron spectra suggests that ionization out of the 3p state ends up with the parent ion, the vibrational energy of which is insufficient for the bond cleavage. On the contrary, by virtue of the additional energy acquired by IC from 3p, the internal energy deposited in 3s is available to break the C–N bond, leading to the fragment ion. The fragmentation is found to occur on the ion surface instead of the Rydberg surface. [ABSTRACT FROM AUTHOR]

Coherent nuclear wavepacket behavior obtained by time-resolved spectroscopy is a good choice to capture the real-time evolution of molecular configuration. Using femtosecond time-resolved photoelectron imaging, we investigate the real-time evolution of the vibrational wavepacket of 2,5-difluoroaniline following the coherent excitation of an out-of-plane vibrational mode in the S1 state at 289.8 nm. Probed by an accidental resonance with the Rydberg states, the periodic oscillations with the frequency of 99 cm−1 are observed from the photoelectron kinetic energy (PKE) distributions, corresponding to the energy difference between the out-of-plane mode X10 of C–F bond and the band origin. Moreover, phase reversal of π rad between 0.66–0.75 and 1.00–1.08 eV is also observed in the PKE region. Combined with the scan of the potential energy surface in the ground cationic D0 state, the observed two ionization channels corresponding to different phases are attributed to the periodic geometry changes between the planar and the non-planar structures when the coherent wavepacket evolves from the initial vertical Franck–Condon region toward the global minimum of the S1 potential energy surface. [ABSTRACT FROM AUTHOR]

Structure rearrangement processes, such as isomerization, are attracting extensive interest as a potential carrier in molecular scale electronics design. UV-light-triggered isomerization of Rydberg-excited propanal with two UV photons has been investigated with time-resolved photoelectron spectroscopy. By following the photoionization from 3s Rydberg states in the time domain, the ultrafast structural evolution and the corresponding photoisomerization dynamics are observed and tracked in real-time. The conversion barrier for isomerization from cis-propanal to gauche isomer is estimated to be about 1500 ± 100 cm−1 experimentally. Both the photoisomerization yield and the conversion rate have shown strong dependence on the excitation energy. It is observed that whether vibration modes are selectively excited or not, cis-to-gauche photoisomerization of propanal in 3s Rydberg state occurs once the excitation energy is higher than the conversion barrier without any vibrational excitation specificity. This yields a powerful approach to studying structural evolution dynamics in large molecules, which may have applications in molecular devices. [ABSTRACT FROM AUTHOR]

[Display omitted] • The ultrafast structural motion associated with charge transfer was measured. • The relaxation dynamics of Rydberg excited molecules was elucidated. • Femtosecond time-resolved photoelectron imaging was applied to track the conformation-dependent charge delocalization process. The ultrafast structural motion linked to the charge transfer process in Rydberg excited N,N,N′,N′-tetramethylmethylenediamine (TMMDA) has been monitored in real time using femtosecond time-resolved photoelectron imaging coupled with quantum chemical calculations. Optical excitation to the 3 s Rydberg state initially populates the charge on one of the two amine groups, resulting in a charge-localized structure in the Franck-Condon (FC) region. As the wavepacket evolves on the 3 s potential surface, the molecular geometry changes with time, leading to the corresponding variation in the charge distribution. The ensuing structural evolution yields two distinct conformers GG+ and TT+ (see text for nomenclature), both with the charge delocalized between the two nitrogen atoms. By virtue of the sensitivity of the Rydberg electron binding energy (BE) on the nuclear geometry, the time-dependent BE spectrum offers an intuitive mapping of the charge transfer reaction that leads from the initially prepared charge-localized GG-FC structure to the fully charge-delocalized GG+ and TT+ structures. Complementary computations provide evidence that through-space interaction is responsible for the charge delocalization in the GG+ and TT+ structures. [ABSTRACT FROM AUTHOR]

Conformation-selective excitation of Rydberg states in iodocyclohexane was operated and the following vibration-specific predissociation dynamics was investigated with excitation of 204.2nm-201.5nm. The timescale of the relaxation processes are measured. It has presented obvious vibration-specific characters, which indicate the multidimensional nature of the predissociation dynamics. [Display omitted] • Iodocyclohexane was conformation selective excited with tunable VUV fs pulse. • Vibration-specific predissociation dynamics of Rydberg states in axial- c -C 6 H 11 I was studied. • The lifetimes of different Rydberg states were measured to be < 1 ps. • Multidimensional nature of the predissociation dynamics was discussed. The predissociation-mediated excited state decays for several vibronic levels of the B and C Rydberg states of axial-iodocyclohexane (axial- c -C 6 H 11 I) are investigated with time-resolved mass spectrometry. The B or C state of axial- c -C 6 H 11 I was selectively excited with tunable VUV femtosecond laser at wavelengths of 204.2 nm-201.5 nm. The following predissociation decay via the crossing between the Rydberg states and the repulsive A-band was observed and the lifetimes were measured to be < 1 ps for different vibronic levels. The vibronic dependence of the B and C states lifetimes is discussed, which provides further evidence of the multidimensional nature of the predissociation in these states. [ABSTRACT FROM AUTHOR]

Relaxation mechanism of S 1 state in UV excited Crotonaldehyde by femtosecond time-resolved photoelectron imaging combined with time-resolved mass spectrometry. In addition, the energy of several Rydberg states was obtained via accidental resonance with these Rydberg states in the process of multi-photon ionization combined with high level TD-DFT quantum chemical calculation. • Internal conversion was observed in the timescale of <100 fs. • The energies of 3 s and 3 p Rydberg states were obtained experimentally. • Quantum chemical calculation was performed on excited states. Real-time evolution of S 1 state crotonaldehyde is tracked by utilizing femtosecond time-resolved photoelectron imaging and time-resolved mass spectroscopy with UV excitation. Our experimental results show that crotonaldehyde molecule has experienced a quick internal conversion to the ground state in an ultrafast timescale of <100 fs following excitation to S 1 state both with 310 nm and 365 nm pumping. In addition, the energy of 3 s and different 3 p Rydberg states of crotonaldehyde were assigned to be 3 s = 6.98 ± 0.11 eV, 3 p y = 7.30 ± 0.08 eV, 3 p x = 7.63 ± 0.04 eV, and 3 p z = 7.79 ± 0.03 eV by combining the multi-peak time-resolved photoelectron spectroscopy and quantum chemistry calculation results. [ABSTRACT FROM AUTHOR]

Ultrafast electronic relaxation processes following two photoexcitation of 400 nm in p-Fluorotoluene (pFT) have been investigated utilizing time-resolved photoelectron imaging coupled with time-resolved mass spectroscopy. Cascaded electronic relaxation processes started from the electronically excited S 2 state are directly imaged in real time and well characterized by two distinct time constants of ~ 85 ± 10 fs and 2.4 ± 0.3 ps. The rapid component corresponds to the lifetime of the initially excited S 2 state, including the structure relaxation from the Franck-Condon region to the conical intersection of S 2 /S 1 and the subsequent internal conversion to the highly excited S 1 state. While, the slower relaxation constant is attributed to the further internal conversion to the high levels of S 0 from the secondarily populated S 1 locating in the channel three region. Moreover, dynamical differences with benzene and toluene of analogous structures, including, specifically, the slightly slower relaxation rate of S 2 and the evidently faster decay of S 1 , are also presented and tentatively interpreted as the substituent effects. In addition, photoelectron kinetic energy and angular distributions reveal the feature of accidental resonances with low-lying Rydberg states (the 3p, 4s and 4p states) during the multi-photon ionization process, providing totally unexpected but very interesting information for pFT. [ABSTRACT FROM AUTHOR]

[Display omitted] • There exist three relaxation pathways following the excitation on the S 1 state. • The attribution of component with hundreds fs is explored. • The intersystem crossing and decay of triplet states are observed. • The accidental resonance with intermediate Rydberg states is discussed. The ultrafast dynamics of 3,4-difluoroaniline is studied by time-resolved velocity map imaging and mass spectroscopy. Following the excitation with 299.6 or 291.8 nm to the S 1 state, relaxation process with three decay components is explored. The hundreds-of-picosecond decay is attributed to the intersystem crossing to triplet state and the nanosecond decay represents the following relaxation of the triplet state. However, the dynamics in hundreds-of-femtoseconds for two excitations are different. We intend to designate it as the molecules leaving the Franck-Condon region at 299.6 nm and as the intramolecular vibrational energy redistribution at 291.8 nm with highly vibrational excitation. [ABSTRACT FROM AUTHOR]