Abstract
Experimental studies of
phenylacetylene (PA) in the gas phase previously revealed an unusual new
set of photophysical properties following the S1←S0
excitation. The decay of the initially excited level showed the expected
short (75ns) lifetime component assigned to the S1 state and a second
very long lifetime (>100 microsec) component that were both created
during the excitation laser pulse [Hofstein J., Xu H., Sears T. J.,
Johnson P. M., J. Phys. Chem. A 112 (2008)]. To investigate the
identity and formation mechanism of the long-lived species, a good
understanding of the vibrational structure in the S1←S0
spectrum of PA is required. The interaction of the vibrational and
electronic (vibronic) motions was calculated in order to predict the
S1 and ←S0 electronic spectrum of PA using a combination of
time-dependent density functional theory (TD-DFT) electronic structure
codes, Franck-Condon integral calculations, and a second order vibronic
model developed previously [Johnson P. M., Xu H. F., Sears T. J., J.
Chem. Phys. 125 (2006)]. The sensitivity of the calculations to
different DFT functionals (B3LYP, LC-BLYP and CAM-B3LYP) and basis sets
(aug-cc-pVDZ and aug-cc-pVTZ) was explored and compared to accurate
experimental relative intensities of the vibrational bands up to
energies of 2000 cm-1 above the band origin obtained by recording the
S1(A 1B2) and #61663; S0(X 1A1) electronic transition using
resonance-enhanced multiphoton ionization spectroscopy (1+1 REMPI) in a
molecular beam and cavity-ringdown (CRD) absorption spectroscopy in a
slit jet-cooled sample. The long-range corrected functional CAM-B3LYP
resulted in a 5 times bigger equilibrium transition moment than B3LYP, a
result more closely matching the observations. The LC-BLYP result is yet
36% higher than CAM-B3LYP. It was observed that CAM-B3LYP produced a
good match to most of the experimental spectrum. However, a superior
match was observed when the CAM-B3LYP equilibrium transition moment and
the moment derivatives from the B3LYP calculation were combined in the
simulation. Results from the CRD experiment showed errors in relative
intensities derived from the REMPI spectra, showing that REMPI
spectroscopy is a less than accurate technique in this regard. Further
pump-probe photoionization experiments not only confirmed Hofstein et
al.'s observations but showed that some of strong assigned
vibrational modes (ΝΒcc and Ν15) enhanced the formation of
the long-lived species compared to others. The motion of the atoms in
ΝΒcc and Ν15, along with the computational analysis of the
energies of electronic states of different PA isomers with respect to
the S1 state of PA., suggest isomer formation as a possible explanation
for the observed long-lived species. The triplet ground state of
Cycloocta-1,3,5-trien-7-yne (COTY), for example, appears to be a good
candidate However, the evidence is still far from conclusive and
additional experimental work is required to investigate the intriguing
complexity in the photophysics of PA.
Description
204 pg.
