Haisheng Liu

The T1(nπ*)←S0 laser induced phosphorescence excitation spectrum of acetaldehyde in a supersonic free jet: Torsion and wagging potentials in the lowest triplet state

The laser induced T1(nπ*)←S0 phosphorescence excitation spectrum of jet‐cooled acetaldehyde has been observed for the first time with a rotating slit nozzle excitation system. The vibronic origins were fitted to a set of levels that were obtained from a Hamiltonian that employed flexible torsion‐wagging large amplitude coordinates. The potential surface extracted from the fitting procedure yielded barriers to torsion and inversion of 609.68 and 869.02 cm−1, respectively. Minima in the potential hypersurface at θ=61.7° and α=42.2° defined the equilibrium positions for the torsion and wagging coordinates. A comparison to the corresponding S1‐state parameters showed that the torsion barrier (in cm−1) does not greatly change, S1/T1=710.8/609.7, whereas the barrier height for the wagging‐inversion barrier increases dramatically, 574.4/869.0.

Toluene internal-rotation: Measurement and simulation of the high-resolution S1–S0 fluorescence excitation spectrum at 0.5 K

Rotational structure in the origin band of the S1–S0 fluorescence excitation spectrum of toluene has been measured at 0.012 cm−1 resolution with a rotational temperature of 0.5 K using a pulsed beam apparatus. Such spectra have been obtained for the parent isotopomer and for the isotopomers with mono- and di-deuterated methyl groups. These, and previously known forbidden bands in which the internal-rotation quantum number Ki changes, are simulated here using ab initio internal-rotation-angle-dependent geometries, potential functions, and electronic transition moment function. An adjustment of some of the ab initio parameters allows a close fitting of the spectra to be made, and this can only be achieved if bond lengths and angles are allowed to vary with internal rotation. The resulting geometries for ground and excited-state toluene are the most accurate to date.

The cavity ringdown spectrum of the visible electronic system of thiophosgene: An estimation of the lifetime of the T1(ã 3A2) triplet state

To obtain insights into the photophysical properties of collision-free T1(a 3A2) thiophosgene, Cl2CS, the cavity ringdown (CRD) spectrum of the T1←S0 absorption system was recorded under supersonic jet conditions and compared with the corresponding excitation spectrum of the total emission. It was found that none of the T1←S0 bands in the CRD spectrum appears in the excitation spectrum, indicating that the T1 thiophosgene decays almost exclusively by the nonradiative T1→S0 intersystem crossing (ISC). An estimation of the T1 nonradiative lifetime was made using the T1–S0 spin-orbit coupling and the Franck–Condon factors for the T1→S0 ISC based on the ab initio equilibrium structures and vibrational frequencies for the T1 and S0 states, computed at the MP2/6-31G(d,p) and MP4/6-31G(d,p) level of theory. The nonradiative life, calculated as the 1/e of the survival probability, is approximately 20 ps for barrier heights of 770–845 cm−1 and out-of-plane angles of 32.07°–32.69°. (The thiophosgene adopts a pyrami...

The hybrid rotational components in the 000 origin band of the ÃA‘(S1)←X̃A’(S0) transition in acetaldehyde

The 000 origin band of the S1←S0, electronic transition that results from n→π* electron promotion has been observed under molecular beam conditions with a pulse amplified ring laser. At low temperatures, ∼0.7 K, the spectrum consisted of 11 lines that originated from Ka‘=0 and J‘=0 or 1 rotational levels. A rotational analysis revealed that the transition between the a1–a1 torsional levels gives rise to a c‐type band, whereas the e–e levels are connected by a hybrid transition that has components along the a, b, and c principal axes. The fluorescence emission from the e levels was greatly reduced at temperatures above 3 K. The interpretation of this photophysical effect requires an intermolecular collision within the molecular beam that quenches the fluorescence from the S1 state.

The phosphorescence excitation spectrum of biacetyl: An analysis of the acetyl and methyl torsional mode structure

The T1(n,π*)←S0 laser-induced phosphorescence excitation spectrum of biacetyl has been recorded with a rotating slit nozzle excitation apparatus. The 000 system origin was observed for the first time as a weak band at 19 529 cm−1. Built on this band were the activities of the three large amplitude modes: υ16(au) acetyl torsion, υ15(au) gearing methyl torsion, and υ21(bg), the antigearing methyl torsion. The potential surface for methyl internal rotation extracted from fitting the observed levels yielded a barrier to methyl torsion of 359.6 cm−1.

Magnetic field effects on the single‐rovibronic‐level fluorescence of S1(1B1) pyrimidine: Study of the singlet–triplet coupling by level anticrossings and quantum beats

Level anticrossings and quantum beats have been observed for the fluorescence from single rotational levels of the vibrationless S1 pyrimidine in a supersonic free jet. The density of triplet levels (ρ) and the coupling matrix elements (v) deduced from the data obtained for R(0) transition confirm that the singlet–triplet coupling belongs to the weak coupling limit (vρ≪1) of interstate interaction, in which electronic relaxation from the prepared singlet state to the triplet manifold does not occur in the absence of collisions.

Rotational symmetry control of electronic relaxation in ultracold gaseous molecules: Optical–optical double resonance probe of the forbidden T1→S0 intersystem crossing in jet-cooled thiophosgene, Cl2CS

Two-color optical–optical double resonance studies of the T1→S0 intersystem crossing in jet-cooled thiophosgene indicate that rotational symmetry strongly influences the efficiency of an intersystem crossing in symmetric molecules with low degrees of rotational excitation.

Torsional symmetry dependence of S{sub 1} dynamics in molecules that under methyl internal rotation

the single-rovibronic-level fluorescence of intermediate-case molecules that undergo methyl internal rotation is strongly influenced by the torsional symmetry of the lowest excited singlet state (S{sub 1}). The most dramatic example of such symmetry dependence comes from the recent finding that the intensities of the e-e transitions in the high-resolution S{sub 1} {l_arrow} S{sub 0} fluorescence excitation spectra of jet-cooled acetaldehyde become very weak relative to the a-a transitions at higher beam temperatures. In this study, they rationalize this remarkable torsional symmetry dependence of electronic relaxation in acetaldehyde on the basis of internal-overall rotation coupling that leads to symmetry-selective increase in the density of states for singlet-triplet coupling. Related observations by others on aliphatic carbonyls and diazabenzenes are also discussed within the context of the coupling between the internal and overall rotation.

PHOTOIONIZATION PROBE OF CHAIN CONFORMATIONS IN SUPERSONIC JET : CONFORMATION-SPECIFIC TWO-PHOTON IONIZATION IN 1,3-DIPHENYLPROPANE

Abstract We report here highly conformation-selective, one-color resonant, two-photon ionization for jet-cooled 1,3-diphenylpropane, which illustrates the utility of the method as a probe of chain conformations in bichromophoric systems.