Stacey A. Wittmeyer

Spectral hole burning in free perylene and in small clusters with methane and alkyl halides

Abstract Pump-probe experiments have been used to sample hole-burning phenomena in the spectra of molecular clusters isolated under supersonic-jet conditions. By using two dye-laser pulses of −1 bandwidth, spaced by ≈60 ns, the spectra of different components of a supersonic expansion can be readily separated. In particular, details such as low-frequency mode structure can be examined for different structural isomers at the same level of aggregation. Data are given for methane/perylene clusters up to 3:1 and for different isomers of perylene/1-chlorobutane at the 1:1 level of aggregation.

Mode- and complexation-specific singlet-triplet coupling in jet-cooled perylene studied by time-resolved fluorescence spectroscopy

Abstract Fluorescence spectral data for jet-cooled perylene indicate little or no vibrational coupling for most levels up to 1057 cm −1 . Fluorescence time profiles have been measured for several levels, and evidence presented for several cases of coupling to the triplet manifold. The principal Franck-Condon-active mode 353 1 exhibits a single-exponential decay, although the emission spectrum indicates little or no singlet vibrational level mixing. The (CH 4 ) 1 complex of perylene, also excited into 353 1 , exhibits a single-exponential profile with a fast decay, again indicating strong singlet-triplet coupling, while the 2 : 1 and 3 : 1 complexes at the same energy are strongly fluorescent. These results imply that singlet-triplet coupling is level-selective, and can be induced by complexation.

Time-resolved desorption of argon and methane from the surface of a perylene molecule

Abstract Time-resolved fluorescence measurements have been made of the predissociation of van der Waals complexes of perylene with Ar and CH 4 . At 705 cm −1 , predissociation of Ar 1,2 complexes occurs in competition with efficient singlet-triplet coupling. At 900 cm −1 , both Ar and CH 4 complexes show little contribution from singlet-triplet coupling. The predissociation of CH 4 complexes at this energy is very rapid, largely as a result of extensive vibrational coupling.

Weak vibrational coupling in a large van der Waals complex: Fluorescence spectroscopy of perylene/naphthalene

Fluorescence excitation and dispersed fluorescence spectra are reported for 1:1 and 2:1 complexes of naphthalene with perylene under supersonic jet conditions. Confirming preliminary results, the fluorescence spectra of the 1:1 complex following excitation of an ag (in‐plane) mode at 353 cm−1 and its first overtone show unusually weak vibrational coupling. Although excitation of combination levels of 3531 with out‐of‐plane modes at 74, 79, and 93 cm−1 gives rise to emission which is predominantly ‘‘relaxed,’’ the residual ‘‘unrelaxed’’ component indicates a significant degree of mode‐selective vibrational coupling. It is notable that the vibrational coupling for 3532 excitation (i.e., at ≈700 cm−1) is substantially less extensive than for excitation into the 3531 combination bands nearly 300 cm−1 lower in energy. A similar comparison has been made between a second ag mode, at 550 cm−1, and a perturbed b3g (out‐of‐plane) mode, at 540 cm−1. In this case, the data indicate a difference in coupling, which is ...

Vibronic hole-burning spectroscopy of perylene/CO2 clusters: intermolecular coupling effects

Abstract Two-photon pump—probe experiments involving absorption—depletion (hole-burning) spectroscopy have measured the vibronic spectra of CO 2 complexes with perylene. Four types of Franck—Condon active mode are indicated by the spectrum of the 1:1 aggregate: (a) Many of the totally symmetric (in-plane) modes of perylene are only weakly perturbed by CO 2 complexation. (b) Certain out-of-plane modes are significantly perturbed, including changes in both Franck—Condon factor and frequency. (c) Low-frequency progressional structure due to intermolecular vibrational modes indicates an unstable conformation for the CO 2 complexes, which can be attributed to multipolar interactions. (d) The spectrum of perylene/(CO 2 ) 1 at higher internal energies exhibits doubling of some progressions.

Picosecond time resolution of the S2 fluorescence of jet-cooled xanthione

Abstract The fluorescence lifetime of the S 2 electronic level of xanthione has been measured under supersonic jet conditions to be 345±10 ps. Fluorescence lifetimes ranging from 240 to 370 ps were also measured, following excitation into different S 2 vibrational levels. This confirms that, in addition to a well known solvent and internal CH mode dependence, the non-radiative coupling of S 2 to lower electronic states is also a function of the vibrational mode excited.

Picosecond time resolution of the S2 state of xanthione in 1 : 1 van der Waals complexes

Abstract Picosecond time-resolved fluorescence measurements, supported by hole-burning spectroscopy, were used to determine the S 2 lifetime of xanthione complexed with individual n -alkane molecules under supersonic jet conditions. Following excitation via the 0 0 0 transition, the fluorescence lifetimes are reduced by up to 60% by alkane complexation, displaying some interesting parallels with the dynamics observed in fluid solution at room temperature. The iso-octane and fluorocarbon complexes show rather longer lifetimes (300 ps or greater) than n -alkanes (200–220 ps), while the cyclohexane complex shows a significantly shorter fluorescence decay time (145 ps). Comparison with the excitation spectra indicates that non-radiative interaction between the complexing species and xanthione depends on the (zero-point) structure of the complex. At higher internal energies, where conformational relaxation (large-amplitude motion) is possible, the distinctions between different alkane complexes are significantly reduced. This kind of experiment provides an important means to study the role of intermolecular interactions in non-radiative processes and unimolecular photochemistry.

Aromatic molecular clusters and their predissociation dynamics studied by picosecond time‐resolved fluorescence spectroscopy

One‐photon resonant, two‐photon ionization experiments are capable of determining the masses of simple molecular complexes, provided that the system has low internal energy. Yet, where larger amounts of internal energy are involved, so that cluster fragmentation can take place, the route by which a system arrives at ionized photofragments is often not clear. In view of the lack of experimental information, the current work has used time‐resolved fluorescence spectroscopy to determine the vibrational predissociation dynamics of neutral complexs of the aromatic species perylene. The results show clearly that simple argon complexes, for internal energies 〈2× the binding energy, generally predissociate on a nanosecond time scale. On the contrary, methane complexes involving similar amounts of internal energy are found to undergo predissociation in the picosecond domain, as a result of efficient internal coupling of vibrational energy.