Xiaofeng Tan

Cavity ring-down spectroscopy and vibronic activity of benzo[ghi]perylene.

Gas-phase cavity ring-down spectroscopy of jet-cooled benzo[ghi]perylene (C22H12) in the 26 950-28 600-cm(-1) spectral range is reported for the first time. This study is part of our extensive laboratory astrophysics program for the study of interstellar polycyclic aromatic hydrocarbons. The observed spectrum shows an intermediate level structure and significant broadening and is associated with the vibronically coupled S1(1A1)<--S0(1A1) and S2(1B1)<--S0(1A1) electronic transitions. Time-dependent density-functional calculations were performed to calculate the energetics, vibrational frequencies, and normal coordinates of the S1 and S2 states. A simple vibronic model was employed to account for the vibronic interaction between the vibronic levels of the S1 and S2 states. The calculated vibronic spectrum is found to be in good agreement with the experimental spectrum.

Experimental and theoretical investigation of the rotational structure of the Al–H2/D2 complex

The rotational structure of the Al–H2/D2 complex is investigated in a collaborative experimental and theoretical study. The isotopomeric complexes were prepared in a pulsed supersonic beam, and their 3d←3p and 4p←3d electronic transitions recorded through laser fluorescence excitation spectroscopy. Transitions to quasibound excited vibronic levels were observed by monitoring emission from lower excited Al atomic levels, formed by nonradiative decay of the excited complex. In some bands, the Lorentzian width was sufficiently narrow that resolved rotational lines were observed. Rotational analysis of several bands which typify the two different patterns of observed rotational structure is presented. The derived rotational constants and parity splitting parameters for the ground Al(3p)–oH2/pD2 bend-stretch levels were compared with constants computed from fits to J- and parity-dependent energies calculated with new Al(3p)–H2 potential energy surfaces (PESs), which extend those recently reported by Williams a...

Experimental and theoretical investigation of the AlH b 3Σ−–a 3Π electronic transition

The laser fluorescence excitation spectrum of the b 3Σ−–a 3Π (0,0) band of AlH and AlD is reported. The AlH/AlD(a 3Π) state was prepared in a free-jet supersonic expansion by the reaction of photolyzed trimethylaluminum with hydrogen or deuterium. Spectroscopic constants for the upper and lower vibronic levels were derived from fits to the measured transition wave numbers of the rotational lines. Lifetimes of J′=1 rotational/fine-structure levels of electronically excited AlH/AlD(b 3Σ−,v′=0) were determined from fluorescence decay waveforms with laser excitation on isolated rotational lines. The measured lifetimes were compared with values obtained in a theoretical treatment of the excited-state decay dynamics, wherein both radiative decay to the a 3Π state and nonradiative decay through the repulsive 1 3Σ+ state were considered. The experimental and theoretical lifetimes are in good agreement. The theoretical treatment shows that the nonradiative excited-state decay dominates over radiative decay. The ob...

Electronic spectroscopy and excited state dynamics of the Al–H2/D2 complex

The electronic spectra of the Al-H2 and Al-D2 complexes are investigated in a collaborative experimental and theoretical study. The complexes were prepared in a pulsed supersonic beam and detected with laser fluorescence excitation spectroscopy. Transitions to bound vibrational levels in electronic states correlating with the excited-state Al(3d, 4p, 4d) + H2/D2 asymptotes were observed by monitoring emission from lower excited Al atomic levels, formed in the non-radiative decay of the excited complex. Fluorescence depletion has also been used to verify that the observed Al-H2 bands all involve the same molecular carrier. The bands have been assigned to the more strongly bound Al-oH2 and Al-pD2 nuclear spin modifications. In contrast to our previous observations for Al(5s)-H2 [X. Yang and P. J. Dagdigian, J. Chem. Phys., 1998, 109, 8920], for which only one potential energy surface (PES) emanates from the dissociation asymptote, the Lorentzian widths of the different vibrational bands in the 3d, 4p, 4d<--3p transitions vary widely, in some cases allowing resolution of the rotational structure of the bands. With the help of the calculated Al(3p)-oH2/pD2 dissociation energies, binding energies of the observed excited vibronic levels are reported. The mechanism of predissociation is investigated theoretically through ab initio calculation of C2 nu cuts of the excited PESs. It is concluded that predissociation occurs through coupling with the repulsive Al(4s)-H2 PES. With these calculations, a qualitative interpretation of the observed bands could be made.

Observation of new electronic states of the Al–H2/D2 complex

Abstract A study of the electronic spectra of the weakly bound Al–H 2 and Al–D 2 complexes is presented. The complexes were generated in a pulsed, free-jet supersonic beam and detected with laser fluorescence excitation and depletion spectroscopy. All observed excited vibronic levels predissociate, and the transitions were detected by observation of lower-energy, emitting Al atoms. A long excited-state progression in the Al–H 2 /D 2 stretch vibrational mode associated with the strongly bound 2 B 2 electronic state correlating with the Al(3d) atomic asymptote was observed. The Lorentzian widths of these bands are large, indicative of strong coupling to the repulsive Al(4s)–H 2 state. A Franck–Condon analysis was carried out to derive an effective one-dimensional potential energy curve for the van der Waals stretch coordinate in the excited 2 B 2 state. This potential energy curve is compared with the previously computed one-dimensional C 2v cut for this state [Faraday Discuss. 118 (2001) 387]. Bands associated with the mixed 5d, 6s←3p electronic transition are also reported. A complex of pattern of vibronic energies was observed, and the Lorentzian widths of these bands vary greatly. The binding energies of these and other Al–H 2 Rydberg states are compared with the computed binding energy of the ionic Al + –H 2 complex.