G. Zerza

The visible emission and absorption spectrum of C60

The fluorescence spectrum of C60 in Ne and Ar matrices is revisited in the light of improved calculations of vibronically induced intensities for the transitions between S0 and the lowest three singlet excited states. The Jahn–Teller and Franck–Condon vibronic activities for the three transitions are also evaluated and combined with the computed Herzberg–Teller intensities to simulate the fluorescence spectra. The agreement between experimental and simulated spectra is very satisfactory and provides an assignment for almost every observed band. The excitation spectrum of C60 in Ne and Ar matrices in the 640–500 nm region is also presented. It is almost a mirror image to the fluorescence spectrum of C60 in Ar matrices and nearly all bands can be attributed by analogy with the fluorescence spectrum. However, it appears that some mode frequencies increase while others decrease as compared to the fluorescence spectrum. In the high-frequency region evidence for a number of bands whose intensity has no counterp...

Absorption wavelengths and bandwidths for interstellar searches of C60 in the 2400-4100 .ANG. region

Low-temp. gas-phase wavelengths and bandwidths for astronomical searches for C60 in the 2400-4100 .ANG. region are detd. from the absorption spectra of C60 in rare gas matrixes at 4 K, coupled with information on the dependence of the vibronic transition frequencies of this fullerene on the Lorentz-Lorenz polarizability parameter of matrixes and solvents. These bandwidths increase from 25 +- 5 cm-1 for the 1 1T1u-1 1Ag band at 4024.0 +- 0.5 .ANG. to 2300 +- 200 cm-1 for the 6 1T1u-1 1Ag band at 2488 +- 8 .ANG.. Spectral observations of astrophys. objects considered to be likely sites of C60 were examd. in the context of the wavelengths and bandwidths of bands of the allowed transitions of C60 which we recommend for searches of this species. No certain pos. assignment of a C60 band was made in the 2400-4100 .ANG. spectral region of these objects. The phys. origin of the obsd. C60 absorption bandwidths is analyzed and discussed in an appendix. [on SciFinder (R)]

Fluorescence spectra of isolated molecules in neon and argon matrices: assignment of the lowest emitting states

Highly structured fluorescence is reported for C60 mols. isolated in Ne and Ar matrixes. Their excitation spectra are similar to the gas-phase 2-photon ionization spectrum of C60. Most of the bands seen in emission can be attributed to Herzberg-Teller or Jahn-Teller active modes and to combinations of these modes. The intensity distribution of the fluorescence bands is compared with calcd. intensities and shows that the emission is of mixed singlet T1g and T2g character in Ne matrixes in nearly equal proportions and of mixed singlet T1g and Gg character in Ar matrixes, in a proportion of 4 to 1 in favor of the T1g state. The band frequencies show that these 3 states are degenerate to within exptl. uncertainty and the authors est. their gas-phase energy at 15,695 +- 5 cm-1. The assignment of ground-state vibrational band frequencies are rediscussed. [on SciFinder (R)]

Phosphorescence of C60 in rare gas matrices

Abstract The phosphorescence of C 60 in Kr and Xe matrices is reported. It consists of a progression of bands of monotonically decreasing intensity which belong to the h g (1) mode but in Xe weaker contributions from the t 1u , t 2u , h u and g u modes are also present. In Xe matrices, the phosphorescence stems from a distribution of sites extending up to 150 cm −1 to the red of the main and most prominent one having its origin at 12714 cm −1 . The phosphorescence lifetime is 16 ± 1 μ s in Xe and 60 ± 5 μ s in Kr. The phosphorescence is attributed to the 3 T 2g state, whose gas phase energy we estimate to be 12925 ± 10 cm −1 from the gas-to-matrix shift.

Lifetime lengthening of molecular Rydberg states in the condensed phase

We report on fluorescence lifetime lengthenings of molecular Rydberg states in condensed media in the case of the NO molecule trapped in inert gas matrices. In rare gas matrices, the fluorescence of the A 2Σ+ state originates from two types of sites, hereafter called red and main. The red site is considered to be a loose site with more than one vacancy in Ar, Kr, and Xe and an h.c.p. site in Ne matrices. It exhibits a lifetime lengthening with respect to the gas phase of 25% in Kr matrices and 100% in Xe matrices. The main site fluorescence stems from monosubstitutional sites. It exhibits lifetime enhancements of up to 100% when going from Ne to Xe matrices. When, however, the fluorescence quantum yields are taken into account, the lifetime increases from the gas phase value to up to two orders of magnitude in the sequence H2–Ne(D2)–Ar–Kr–Xe. Furthermore, this change in transition moment is not observed in the absorption spectrum. These results stress the influence of the solvent and its microscopic struc...

Picosecond studies of the intramolecular relaxation processes in isolated C60 and C70 molecules

Time-resolved fluorescence spectra for C60 mols. embedded in Ne and Ar matrixes and C70 in Ne matrixes were recorded using a synchroscan streak camera. In the case of C60, the decay times of the S1 state are identical for all fluorescence bands confirming that it is a state of mixed T1g, T2g, and Gg characters. Its lifetime is detd. by the intersystem crossing rate to the triplet manifold which increases in going from Ar to Ne matrixes. This is attributed to a change of the dominant symmetry character of S1 in the 2 matrixes. A transient, short-lived (.apprx.170 ps in Ne, .apprx.70 ps in Ar), S3 fluorescence of dominant Gg symmetry is also obsd., which feeds the S2/S1 population (of dominant T2g and T1g symmetry). Using the Energy Gap Law, a change of the S3 S2/S1 decay rate from Ne to Ar matrixes is attributed to the different no. of phonons needed to bridge the gap between these states and/or to an environment-dependent change of the coupling matrix element. In C70, the fluorescence from both S1 and S2 are populated at the same rate despite the 165 cm-1 energy gap between them. This is explained in terms of an intersystem crossing from the triplet state manifold. Finally, S2 decays predominantly to S1, while the latter decays to the triplet states by reversible intersystem crossing. These conclusions are confirmed by a simple kinetic model. [on SciFinder (R)]

Rydberg series in condensed matter: a fluorescence depletion experiment

Rydberg-Rydberg transitions are induced by absorption from the lowest Rydberg state of NO A 2S+ (3ss) in Ar matrixes, which is excited by an ArF laser at 193.3 nm. The depopulation of the A(v = 0) level to higher Rydberg states is probed by depletion of its fluorescence. Absorption bands are obsd. in the 1.0-2.0 eV region which are attributed to transitions to the 3p, 3d, 4s and 4p members of the Rydberg series. At higher energies, a continuous absorption structure appears. It is attributed to transitions to the n>4 members of the Rydberg series which are compressed in a tiny energy range below the ionization potential. A quantum defect model is applied and accounts well for the observations. [on SciFinder (R)]

Picosecond and femtosecond studies of the energy redistribution in matrix-isolated C60 molecules

The intramol. energy relaxation processes of UV-excited C60 mols. embedded in Ar crystals were studied by picosecond fluorescence studies and femtosecond pump-probe absorption spectroscopy. The various internal energy relaxation steps are clearly identified. The importance of intramol. energy relaxation processes in solid C60 films is underlined. [on SciFinder (R)]

Femtosecond study of the rise and decay of carbenes in solution

Abstract The ultrafast production and decay of carbenes from the diphenyldiazomethane precursor in isooctane has been investigated by femtosecond transient absorption spectroscopy. Diphenylcarbene is produced in less than 300 fs in a singlet state. It then undergoes an ultrafast (∼300 fs) internal conversion followed by an intermediate step of a few picoseconds, which we attribute to a conformational change between its C 1 and C 2v geometries. Finally, intersystem crossing from S 1 to the triplet ground state occurs in ∼100 ps.

Comment on `The dispersed laser induced fluorescence spectrum of gas phase at 308 nm'

In a recent paper by Firth and co-workers, UV emission bands were observed which have been attributed to emission. In the present contribution we show that these bands in fact stem from impurities.