Wm. R. Lambert

Jet spectroscopy of anthracene and deuterated anthracenes

Fluorescence excitation and SVL fluorescence spectra of jet-cooled h10-, 9d1-, 9,10d2-, and d10-anthracene are reported. Ground state vibrational assignments are presented for all these species and are compared with literature values. In addition, assignments for the first excited singlet state of h10-anthracene are made using SVL spectra and rotational band contours as guides. The work presented herein serves as an essential reference for other work from this research group concerning the dynamics of excited anthracene (see accompanying papers), and completes the spectroscopy of the polyacene series.

Picosecond excitation and TRANS-CIS isomerization of stilbene in a supersonic jet: Dynamics and spectra

New details of the dynamics and spectroscopy of trans-stilbene are revealed by picosecond excitation of jet-cooled and collisin-free molecules. The fluorescence spectra clearly show the low-frequency torsional modes of the ground state. The time-resolved fluorescence following S_1 excitation exhibit exponential decay times ranging from 2.7 ns to ≈190 ps for excitation wavelengths varying from 3102 A (0 excess energy) to 2850 A (2851 cm^(−1) excess energy). The results indicate that an energy threshold for isomerization occurs at ≈1200 cm^(−1). The redistribution of vibrational energy from the optically excited modes to the ethylene bond torsional modes leading to isomerization is discussed.

Quantum beats and dephasing in isolated large molecules cooled by supersonic jet expansion and excited by picosecond pulses: Anthracene

The modulation depth and lifetime were measured for quantum beats observed in the fluorescence decay of anthracene. (AIP)

Picosecond excitation of jet-cooled pyrazine: magnetic field effects on the fluorescence decay and quantum beats

Quantum beats and fluorescence decays of the vibrationless S_1 state of jet-cooled pyrazine have been studied as a function of magnetic field. Fourier analysis of the modulated portions of these decays has yielded results which are consistent with first- and second-order Zeeman effects occurring in the excited state. An increase in the number of triplets states effectively coupled to the singlet manifold with increasing magnetic field is discussed in terms of Zeeman interactions.

Picosecond excitation and selective intramolecular rates in supersonic molecular beams. I. SVL fluorescence spectra and lifetimes of anthracene and deuterated anthracenes

Fluorescence spectra and decay rate results for jet-cooled anthracene and some deuterated anthracenes excited to single vibronic levels in their S1 manifolds are presented. Consistent with our quantum beat results (see accompanying paper), fluorescence spectra exhibit the manifestations of negligible, then limited, and finally, extensive intramolecular vibrational energy redistribution (IVR), as the vibrational energy in S1 is increased. The decay rate results indicate that the primary decay channel of the anthracenes for S1 vibrational energies of 0 to 5600 cm^−1 is a radiationless transition involving a small electronic energy gap. These decay rate results also display manifestations of negligible IVR at low energies and extensive IVR at higher energies. The energy at which the decay rates (time-resolved spectra) indicate extensive IVR is about 1800 cm^−1.

Dynamics of intramolecular vibrational-energy redistribution (IVR). IV. Excess energy dependence, t-stilbene

The results of picosecond time‐resolved measurements of intramolecular vibrational‐energy redistribution (IVR) in jet‐cooled t‐stilbene are presented. The results show that the changes in the nature of IVR that were found to occur with increasing energy in anthracene also occur in t‐stilbene. In particular, at intermediate energies a number of different excitations give rise to phase‐shifted quantum beats in fluorescence decays, indicating restricted IVR in the molecule. At higher energies decay behavior characteristic of dissipative IVR is observed. These results are compared with those of anthracene and are discussed in terms of molecular symmetry and vibrational density of states. The results suggest the generality of the conclusions about large molecule IVR that have been stated in the other papers of this series.

Picosecond excitation and selective intramolecular rates in supersonic molecular beams. II. Intramolecular quantum beats and IVR

An account of observations pertinent to quantum beat-modulated fluorescence decays of jet-cooled anthracene excited to S1+1380 cm^−1 is given. Based on both spectral and temporal characteristics of the beats, combined with theoretical expectations of such characteristics, it is shown that the interference phenomenon arises from the coupling of zero-order vibrational levels within the S1 manifold. (Consistent with this interpretation is the observed absence of a magnetic field effect on the beats.) As such, it is shown to be a manifestation of restricted IVR. The results of measurements of rotational band contours and the effect of carrier gas on beat parameters are used to assess the role of rotations in the coupling between vibrational levels. Specific vibration–vibration coupling schemes are briefly discussed within the context of the observations.

Picosecond excitation of jet‐cooled hydrogen‐bonded systems: Dispersed fluorescence and time‐resolved studies of methyl salicylate

Long progressions involving frequency intervals of 180 cm−1 are observed in the fluoresence of MS for 3327.5 A excitation. (AIP)

Direct observation of intramolecular energy transfer by selective picosecond laser excitation of a single chromophore in jet-cooled molecules

Here we report on the direct observation of time-resolved formation of a product of intramolecular energy transfer in isolated, jet-cooled A^*-CH_2-CH_2-CH_2-B. The anthracene chromophore (A) was excited by picosecond pulses, and the transfer of energy was probed at various excess energies by observing the emission arising from the exciplex formed between B (dimethyl aniline) and A^*. The threshold is at 3 kcal/mole.

Energy and phase relaxation of phosphorescent F centers in CaO

In this paper we study the temperature‐induced homogeneous broadening of the no‐phonon line in the emission spectrum of the F center in CaO. The linewidth can be fitted to n(n+1), where n is the thermally averaged occupation number of phonons with a frequency of 90 cm−1. The results are characteristic of elastic scattering of pseudolocalized phonons at the defect site. These phonons also appear to dynamically couple the Jahn–Teller components of the F center in the photoexcited 3T1u state and thus give rise to a temperature dependence of the lifetime of this phosphorescent state. Finally, from experiments using laser‐selective excitation it is concluded that the zero‐phonon emission peaking at 571.1 nm does not originate in the F center.