Chong Tao

Fluorescence excitation and emission spectroscopy of the ÃA″1←X̃A′1 system of CHBr

We report fluorescence excitation and emission spectra of CHBr in the 450–750nm region. A total of 30 cold bands involving the pure bending levels 20n with n=2–8 and combination bands 20n301(n=1–8), 20n302(n=1–6), 20n303(n=1–2), 10120n(n=5–7), 10120n301(n=4–6), and 10120n302(n=5) in the AA″1←XA′1 system were observed, in addition to a number of hot bands. The majority of these are reported and/or rotationally analyzed here for the first time. Spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotational analysis yielded band origins and rotational constants for both bromine isotopomers (CHBr79,CHBr81). The derived AA″1 vibrational intervals are combined with results of [Yu et al. J. Chem. Phys. 115, 5433 (2001)] to derive barriers to linearity for the 2n, 2n31, and 2n32 progressions. The AA″1 state C–H stretching frequency is determined here for the first time, and the observed ν3 dependence of the Br79–Br81 isotope splitting in the AA″1 state is in good agreement ...

First observation of the elusive iodocarbene: ground state multiplicity and singlet–triplet gap of CHI

We report the first observation of iodocarbene (CHI) using fluorescence excitation and emission spectroscopy, which indicate a singlet ground state and a lower bound to the singlet-triplet gap of 3.76 kcal mol(-1).

Spectroscopy and dynamics of the predissociated, quasi-linear S2 state of chlorocarbene

In this work, we report on the spectroscopy and dynamics of the quasi-linear S(2) state of chlorocarbene, CHCl, and its deuterated isotopologue using optical-optical double resonance (OODR) spectroscopy through selected rovibronic levels of the S(1) state. This study, which represents the first observation of the S(2) state in CHCl, builds upon our recent examination of the corresponding state in CHF, where pronounced mode specificity was observed in the dynamics, with predissociation rates larger for levels containing bending excitation. In the present work, a total of 14 S(2) state vibrational levels with angular momentum l = 1 were observed for CHCl, and 34 levels for CDCl. The range of l in this case was restricted by the pronounced Renner-Teller effect in the low-lying S(1) levels, which severely reduces the fluorescence lifetime for levels with K(a) > 0. Nonetheless, by exploiting different intermediate S(1) levels, we observed progressions involving all three fundamental vibrations. For levels with long predissociation lifetimes, rotational constants were determined by measuring spectra through different intermediate J levels of the S(1) state. Plots of the predissociation linewidth (lifetime) vs. energy for various S(2) levels show an abrupt onset, which lies near the calculated threshold for elimination to form C((3)P) + HCl on the triplet surface. Our experimental results are compared with a series of high level ab initio calculations, which included the use of a dynamically weighted full-valence CASSCF procedure, focusing maximum weight on the state of interest (the singlet and triplet states were computed separately). This was used as the reference for subsequent Davidson-corrected MRCI(+Q) calculations. These calculations reveal the presence of multiple conical intersections in the singlet manifold.

Optical-optical double resonance spectroscopy of the quasi-linear S2 state of CHF and CDF. I. Spectroscopic analysis

In this work, we report on our full results of the spectroscopic analysis of the quasi-linear S(2) state of the prototypical halocarbene, CHF, and its deuterated isotopomer CDF using optical-optical double resonance spectroscopy through the S(1) state. A total of 51 S(2) state vibrational levels with angular momenta in the range [script-l] = 0-3 were observed for CHF, and 76 levels for CDF. Progressions involving all three fundamental vibrations were observed, and rotational constants were determined for each of these levels by measuring spectra through different intermediate J levels of the S(1) state. Our experimental results are in excellent agreement with the predictions of vibrational calculations using the discrete variable representation method. The variational vibrational calculations were performed with an analytic potential energy surface fit to ab initio data by the method of interpolating moving least squares. The ab initio data are Davidson-corrected multi-reference configuration interaction calculations based on a state-averaged multiconfigurational self-consistent field reference incorporating a generalized dynamic weighting scheme.

Unraveling the Ã1B1 ← X̃1A1 Spectrum of CCl2: The Renner−Teller Effect, Barrier to Linearity, and Vibrational Analysis Using an Effective Polyad Hamiltonian

We report studies aimed at unraveling the complicated structure of the CCl 2 A (1)B 1 <-- X (1)A 1 system. We have remeasured the fluorescence excitation spectrum from approximately 17,500 to 24,000 cm (-1) and report the term energies and A rotational constants of many new bands for both major isotopologues (C (35)Cl 2, C (35)Cl (37)Cl). We fit the observed term energies to a polyad effective Hamiltonian model and demonstrate that a single resonance term accounts for much of the observed mixing, which begins approximately 1300 cm (-1) above the vibrationless level of the A (1)B 1 state. The derived A (1)B 1 vibrational parameters are in excellent agreement with ab initio predictions, and the mixing coefficients deduced from the polyad model fit are in close agreement with those derived from direct fits of single vibronic level (SVL) emission intensities. The approach to linearity and thus the Renner-Teller (RT) intersection is probed through the energy dependence of the A rotational constant and fluorescence lifetime measurements, which indicate a barrier height above the vibrationless level of the X (1)A 1 state of approximately 23,000-23,500 cm (-1), in excellent agreement with ab initio theory.

Electronic spectroscopy of the ÃA″1↔X̃A′1 system of CDBr

We report fluorescence excitation and single vibronic level emission spectra of jet-cooled CDBr in the 450–750nm region. A total of 32 cold bands involving the pure bending levels 20n with n=3–10 and combination bands 20n301 (n=2–10), 20n302 (n=2–9), 10120n (n=7–10), and 10120n301 (n=6,8–9) in the AA″1←XA′1 system of this carbene were observed; most of these are reported and/or rotationally analyzed here for the first time. Rotational analysis yielded band origins and effective (B¯) rotational constants for both bromine isotopomers (CD79Br and CD81Br). The derived AA″1 vibrational intervals are combined with results of Yu et al. [J. Chem. Phys. 115, 5433 (2001)] to derive barriers to linearity for the 2n, 2n31, and 2n32 progressions. The AA″1 state C–D stretching frequency (2350cm−1) is determined for the first time, in excellent agreement with theory, as are the Br79–Br81 isotope splittings in the excited state. Our emission spectra probe the vibrational structure of the XA′1 and aA″3 states up to ...