Rotationally resolved ultraviolet spectra of benzene-noble gas van der Waals clusters

Sub-Doppler electronic spectra with hundreds of resolved rotational lines are now available for benzene-Ar dimers and trimers. From their analysis the structure of these clusters is precisely determined. The analysis of two bands, 6 ₀ ¹ and 16 ₀ ² , of C₆H₆ · Ar is presented in detail. It leads to accurate values of the van der Waals bond length in the electronic ground and excited state. The change in frequency upon clustering is found to be a factor of 17 larger for the overtone of the out-of-plane modev ₁₆ than for the in-plane vibrationv ₁. This can be tentatively explained by an interaction of the low frequency out-of-plane motion of the ring with the van-der-Waals motion of the Ar atom.     

Lifetime measurements of two-photon excited vibronic levels in the low pressure limit: naphthalene

Single vibronic lifetimes of two-photon excited states are measured in the low pressure limit for naphthalene. The two-photon spectrum, besides yielding new states in S₁, is also free of the interference from the neighborhood of the S₂, known from one-photon experiments, and hence unperturbed lifetimes can be measured to higher excess energies. The overall pattern of lifetimes with excess energy including both one-photon and two-photon states shows that the vibronic structure is subordinate to an overall monotonic decrease, much as found earlier for β-naphthylamine. This indicates, at least for naphthalene, that radiationless processes into triplets are dominated by Franck—Condon factors and not by vibronic inductions and promoting modes.     

Photodissociation of state-selected cluster ions studied by mass-selective, pulsed-field threshold ionization spectroscopy

Delayed pulsed-field ionization of long-lived high Rydberg states yields vibrationally and partly rotationally resolved spectra of polyatomic molecular ions and of cluster ions when the resulting threshold ions are measured as a function of the excitation energy. The field ionized threshold ions are monitored and separated from the non-energy-selected ions in a reflecting field mass spectrometer with high mass resolution. The decay of the molecular or cluster ion core is monitored by the appearance of threshold ions at the daughter ion mass as a function of the selectively excited vibrational state. In this way, upper limits for dissociation thresholds of neutral and ionized dimers are obtained which are smaller than recent theoretical values from the literature. The appearance of daughter fragment ions after delayed pulsed-field ionization indicates that high Rydberg orbits are not destroyed by the dissociation of the core. Possible applications of our technique for the production of state-selected ions are discussed.     

Resolved high Rydberg spectroscopy of polyatomic molecules and van der Waals clusters

Using sub-Doppler double resonance excitation with Fourier-transform limited laser pulses and pulsed field ionization techniques we were able to resolve individual high n Rydberg states (45 < n < 110) below and above the lowest ionization energy of van der Waals clusters of benzene with the noble gases neon and argon. By choosing various selected J'K' intermediate rotational states we detected and assigned several Rydberg series with nearly vanishing quantum defect. They converge to different limits representing the rotational states in the vibrational states of the cluster cation. Even far above the ionization threshold sharp high-n Rydberg states with a width of 750 MHz are observed converging to intramolecular vibrational states located up to 800 cm-1 above the dissociation threshold of the cluster ion. This points to a slow dissociation rate of the cluster ion in the range of 3 x 10(5) s-1 < k < 5 x 10(8) s-1. In further studies single high Rydberg states of benzonitrile, a polyatomic molecule with an high dipole moment of 4.18 D, were detected in the range from n = 50 to 100. We plan to investigate the influence of the strong anisotropic dipole field of this molecule on the coupling between the high Rydberg electron and the molecular core.     

Molecular two-photon spectroscopy with two synchronized tunable dye lasers

Two tunable dye lasers, with differing wavelength and polarization, were synchronized to produce a two-photon spectrum in gas phase benzene. Simultaneous absorption of two photons from one laser beam was precluded by choosing appropriate wavelengths. All possible ratios have been measured for a totally-symmetric two-photon transition demonstrating a strong analogy between two-photon absorption and Raman scattering.     

The temporal development of intracavity absorption in a dye laser

Intercavity absorptions is known to lead to an enormous enhancement in the detectability of absorbing species. Here the time development of this enhancement of this enhancement is investigated in order to fix a typical time scale such measurements. This is done with a Pockles cell in front and inside a flashlamp pumped dye laser resonator. The enhancement of absorption steeply increases at laser threshold and then levels off during the letter part of the laser pulse. By Q-switching a dye laser it is demonstrated that on a time resolution of 20 nsec extinctions as low as 10^-4 can be measured with an enhancement of two orders of magnitude.