A. Tramer

Study of the singlet—triplet coupling in glyoxal by level-anticrossing spectroscopy. I. Experimental techniques and results

Abstract Level anticrossing and a new optical-radiofrequency double resonance technique were applied to a study of the singlet—triplet interactions for single rotational levels of the vibrationless 1Au state of glyoxal. The density and spectral distribution of triplet rotational levels virtually coupled to the (K = 6, J = 13) and (K = 7, J = 35) singlet states were determined. The values of the singlet—triplet coupling constants Vst for some selected level pairs were measured. The assignment to the weak-coupling limit is confirmed.

Fluorescence spectra and intramolecular vibrational redistribution in jet-cooled perylene

Abstract The jet-cooled fluorescence spectra of perylene excited to the S 1 state with E vib = 0–1600 cm −1 are recorded and analyzed. For E vib −1 only the resonant fluorescence was detected. Ground- and excited-state frequencies of 14 low-frequency normal modes are determined. A drastic change in frequency of the “butterfly” modes upon electronic excitation shows that perylene slightly deviates from planarity in its ground state and is more rigid in the excited singlet state. For a number of levels in the E vib = 800–1600 cm −1 range, the fluorescence is composed of the resonant emission and of non-resonant (“‘relaxed’”) bands. It is shown that apparently single bands in the fluorescence-excitation spectrum correspond to ovelapping bands pumping different molecular eigenstates resulting from the intrastate coupling. The relative role of the anharmonicity and of the Coriolis interaction are discussed. The data are treated in terms of a selective coupling between doorway and hallway states with the coupling constant rapidly decreasing with the difference in the overall vibrational quantum number between initial and final state.

A new pulsed light source for lifetime studies and time resolved spectroscopy: the synchrotron radiation from an electron storage ring

Abstract Synchrotron radiation from the Orsay Storage Ring (ACO) has been used as a repetitive source for lifetime and time resolved fluorescence studies. Our measurements and results concern: (1) Lifetime of standard compounds (quinine sulfate and fluorescein anion); (2) single vibronic level fluorescence (aniline and pyrazaine in the vapor phase; pyrazine lifetime has been measured for the first time to be 0.5 ± 0.2 ns); and (3) reactivity of excited molecules (decay time and time resolved spectra of 2-naphthol and fluorescein both undergoing fast protolytic reactions in the excited state). Storage ring synchrotron radiation characteristics and performances are discussed and compared to those of conventional “nanosecond” flash lamps. Possible new applications are foreseen.

On the “third decay channel” and vibrational redistribution problems in benzene derivatives

Abstract Fluorescence lifetimes and yields have been measured for toluene, aniline- h 7 and aniline- d 5 vapors in the spectral range corresponding to a rapid increase of the non-radiative rates. Non-exponential decays and discrepancies between lifetime and yield dependence on ν exc are observed. Results are discussed on the ground of a model assuming: (a) strong anharmonic coupling of “active” and “non-active” modes, (b) specific “third channel” threshold for each vibrational progression and (c) absence of rapid vibrational redistribution.

Selected rotational level lifetimes and singlet-triplet coupling in the S1 state of glyoxal

Abstract The fluorescence collision-free lifetimes were measured for a few rotational levels of the O° S 1 state of glyoxal using a single-mode Ar + laser. Decays are exponential over 5–6 lifetimes and variation of τ 1.k o does not exceed 5%. The higher limit of singlet-triplet mixing coefficient β 2 ⩽ 3 × 10 −2 is deduced from the lifetime measurements. The efficiency of collision induced intersystem crossing and its relation with 2 > value is discussed. ISC is absent in collision-free conditions for deuterated glyoxal as it is known to be for protonated glyoxal.

Vibrational redistribution in jet-cooled perylene intermediate coupling case

Fluorescence excitation spectra (FES) and fluorescence spectra (FS) under single vibronic level excitation of perylene in a supersonic jet are reported. The fluorescence from the lowest vibronic levels of the S1 state is purely resonant while for δEvib > 62; 1600 cm−1 broad-band emission spectra weakly dependent on νexe are explained by vibrational redistribution with a characteristic time 5 <Tred < 100 ps. In the intermediate δEvib range (700–1600 cm−1) complex emission spectra correspond to a strong coupling between the doorway state and a small number of “dark” states. The data are discussed in terms of a model assuming a rapid variation of the anharmonic coupling constant with the overall change of the vibrational quantum number between interacting levels.

Vibrational relaxation by reversible intersystem crossing

Abstract The time- and energy-resolved fluorescence spectra of CO obtained under selective excitation of the A 1 Π (υ′ = 1) level have been studied. The apparent vibrational relaxation in the singlet manifold is shown to be mainly due to the three-step process: the singlet → triplet crossing from υ′ = 1 level, vibrational relaxation in the triplet manifold and reverse crossing to the A 1 Π (υ′ = 0) level. The decay form may be fitted by assuming the relaxation rate constants in triplet (and singlet) manifolds of the order of 10 5 s −1 Torr −1 i.e. smaller by one order of magnitude than previously proposed.

Luminescence and triplet decay in quinoxaline vapors

Abstract The effects of the gas pressure on the quinoxaline triplet state kinetics and luminescence yields were studied in the 10 −3 −10 torr pressure range. The non-exponential character of the fluorescence decay and long fluorescence lifetimes, previously reported, are confirmed. The collision-free lifetime of the “hot” triplet state measured by triplet—triplet absorption kinetics and by phosphorescence collisional induction is of the order of 100 μs, much longer than the long-fluorescence decay times (0.5–70 μs). The results are discussed on the basis of a model involving anharmonic coupling between triplet levels strongly and weakly coupled to the singlet.

Electronic spectra and proton transfer in the phenol/(NH3)n clusters in argon matrices

Abstract Absorption, fluorescence, phosphorescence and fluorescence- and phosphorescence-excitation spectra are recorded for the phenol/ammonia(Φ-OH/NH3) complexes, to Φ-OH..(NH3)2 and large Φ-OH..(NH3)n (n⩾3) clusters are identified. It is shown that the ionic form: Φ-O−..H in very large Φ-OH..(NH3)n clusters formed in pure ammonia and mixed NH3/Ar matrices with a high ammonia content. The results are compared to those obtained for free phenol-ammonia clusters formed in a supersonic expansion.

Site effect on radiative and non-radiative relaxation paths of naphthalene in low-temperature matrices

Abstract Fluorescence and phosphorescence spectra and decay times are recorded for naphthalene in Ne, N 2 , CH 4 , Ar, Kr and Xe matrices upon a selective excitation of different sites. Beside the usual frequency shifts we observe significant differences in the relative intensities of the “allowed” and “forbidden” transitions in the fluorescence spectra and in the S 1 and T 1 decay times (i.e. S 1 ∼→T 1 and T 1 ∼→S 0 intersystem crossing rates) between different families of sites in the same host. These effects are discussed in terms of the medium induced molecular deformations and of the variation of the external heavy atom effect on the S 1 ↔T 1 coupling with position of the perturber.