A.R. Knight

Fluorescence from S2 single vibronic levels in thiophosgene vapor: Quantum yields and quenching effects

The quantum yields of fluorescence from the second excited singlet state of thiophosgene vapor have been measured as a function of pressure for eight S2←S0 transitions, and parameters for the decay of the levels pumped have been calculated. The results indicate that reassignment of the origin of the S2←S0 band system to 35 112 from 36 007 cm−1 is necessary. Quantum yields of fluorescence from the lowest levels in S2 are extremely high (0.50 to 1.0) and are indicative of a very slow radiationless decay rate. Weakening of fluorescence from higher S2 vibrational levels is most likely caused by predissociation.

A spectroscopic study of fluorescence from the second excited singlet state of thiophosgene vapor

The S2–S0 absorption, emission, and excitation spectra of thiophosgene vapor have been measured and the observed single vibronic transitions assigned. Comparison of the absorption and excitation spectra reveals that emission is excited only when transitions to the upper state terminate in 3m4n, m or n=0 or 1, and that the upper state is depopulated by rapid radiationless processes when higher vibrational levels are populated. The most prominent bands in the emission spectrum are those in which the transitions terminate in 1p3q4r with 1⩽p⩽10, 1⩽q⩽5, and 1⩽r⩽4. All three types of spectra are consistent with a pyramidal excited state in which the equilibrium internuclear distances are substantially greater than those of the ground state. The implications of this fluorescence ’’anomaly’’ with respect to the nature of radiationless processes in the thiophosgene are discussed.

Sequential two‐photon absorption and singlet–singlet energy pooling in thiophosgene vapor

Emission from the second excited singlet state of Cl2CS vapor has been monitored following excitation with a tightly focused, pulsed dye laser in the region of the ground state to first excited singlet and triplet absorption systems. Excitation in the triplet bands produces prompt fluorescence from S2 as a result of sequential two‐photon absorption, S0→hνT1→hνS2. Excitation in the bulk of the singlet bands produces no prompt fluorescence but does produce delayed fluorescence as a result of a bimolecular singlet–singlet energy pooling process, 2S1→S2+S0.

Structures and decay dynamics of thiocarbonyl excited states

Abstract Gas phase absorption, emission and emission excitation spectra have provided detailed ground and excited state structural information for several representative small thiocarbonyls. The ground states are planar but the excited states are generally non-planar and may exhibit a large variation in inversion barrier heights and equilibrium out-of-plane angles. The CS bond is elongated in excited states, by as much as 0.5 A in the S 2 1 (π,π*) states of Cl 2 CS and ClFCS. Tetra-atomic thiocarbonyls appear to decay solely by radiative means from bound levels of their S 1 1 (n,π*) states under collision-free conditions; radiative decay of the corresponding T 1 states has not been observed. Excitation to low-lying vibrational levels of the S 2 states of ClFCS and Cl 2 CS in the gas phase results in S 2 → S 0 emission with high quantum yields (ϕ f → 1.0). At higher vibrational energies non-radiative decay (probably leading to dissociation) dominates. The observation of intense emission from S 2 has been exploited to probe the mechanisms of the two-photon absorption in gas phase Cl 2 CS. Sequential two-photon absorption via T 1 and bimolecular singlet—singlet energy pooling have been observed.

Photophysics of the B̃ 1A1 state of thiophosgene

Abstract The results of fluorescence lifetime measurements using a dye laser excitation source are combined with other recently published data to present a consistent picture of the photophysics of the B 1 A 1 state of Cl 2 CS.

Second excited singlet state emission spectroscopy of thiocarbonyls. I. Thiophosgene

Abstract Well resolved emission spectra have been recorded after dye laser excitation of low-lying vibronic levels in the B ( 1 A 1 ) state of Cl 2 CS. The effects of substrate pressure, added SF 6 and excitation wavelength on the spectra are reported. A Franck-Condon analysis suggests that the excited state CS bond length is 0.5 A longer than in the ground state.

Pressure effects in the ÃX̃ laser-excited fluorescence spectrum of Cl2Cs

Abstract The intensities of bands in the A  X fluorescence spectrum of thiophosgene, laser-excited at 514.5 nm, have been measured as a function of pressure. Weak, non-resonance bands grow in with increasing pressure, demonstrating the existence of collisional vibrational relaxation. Differential electronic quenching of A states of different vibronic symmetry is indicated.

Second excited singlet state emission spectroscopy of thiocarbonyls. II. Thiocarbonyl chlorofluoride

Abstract Single vibronic level emission spectra have been measured for four excitation wavelengths pumping the second excited singlet state of CIFCS. Only resonance fluorescence is observed. Progressions involving Δv″1 = 0, 1, 2, …, Δv″6 = 0, 2, 4 … dominate. Analysis of the spectra indicates that neither intra- or intermolecular processes lead to odd Δv′6 changes and that collisions in the pure gas lead to electronic quenching of molecules in the excited state.

Laser excitation study of the second excited singlet state photophysics of ClFCS

A tunable dye laser has been used to excite selected single vibronic levels of the ? (1A′) state of ClFCS in the gas phase. Fluorescence (?→?) excitation spectra, quantum yields, and excited state lifetimes have been measured. Radiative and nonradiative decay constants and bimolecular self‐quenching constants have been calculated for each selected state. A model in which a single dissociative vibrational mode provides the sole nonradiative decay mechanism is proposed.

The B̃(1A′) ← X̃(1A′) spectrum of ClFCS

Abstract The vibrational structure of the first strong electronic transition, B 1 A′ ← X 1 A′ , of CIFCS has been analyzed. Four excited state frequencies have been identified. The excited state is nonplanar and has a barrier to inversion of ⪸2000 cm−1 and a CS bond which is about 0.5 A longer than the ground state.