Hole-Burning Studies of the Splitting in the Ground and Excited Vibronic States of Tetracene in Helium Droplets

Abstract

The laser-induced fluorescence spectra of single tetracene (C 18 H 12 ) and pentacene (C 24 H 14 ) molecules embedded in liquid He N droplets (N ˜= 10 4 ) show sharp zero phonon lines (ZPL) (δν e 0.2 cm -1 ), accompanied by weaker phonon wings (PW) on the blue side. The ZPL of tetracene is anomalously split into a doublet with a separation of 1.1 cm -1 , whereas for pentacene, the ZPL is not split. Hole-burning measurements with two pulsed dye lasers and lifetime measurements indicate that inside He droplets the ground and excited states of tetracene are each split into two levels. The splitting is attributed either to the occupation of two nearly equivalent sites by localized helium atoms or to a tunneling of one or two localized helium atoms through the barrier in the double-well potential on the surface of the tetracene molecule.

Electronic spectroscopy of 9,10-dichloroanthracene inside helium droplets

“…This effect differs from what was reported as line broadening for large amplitude modes such as the butterfly mode of pentacene reported by Hartmann et al 14 and confirmed by Callegari and Ernst. 9 Most effective for line broadening appears to be a change of the molecular permanent electric dipole moment, which determines the induction interaction between a neutral molecule and the helium environment.…”

Absorption spectroscopy of adenine, 9-methyladenine, and 2-aminopurine in helium nanodroplets

“…Although splitting of ZPL's has been observed in many molecular systems, a complete understanding of the phenomena is still lacking. 52,[65][66][67] Analysis of the rotational band contour of the A 2 resonance of gas phase adenine is consistent with a transition involving a 1 pp* state, 14 thus corroborating its previous assignment to the 0-0 transition to the 1 L b (pp*) state. 12 In helium droplets, this transition has the largest linewidth, 12.2 cm À1 , of the observed transitions.…”

“…In order to allow for a quantitative analysis of this effect and an accurate determination of the transition frequencies, we have fitted the lineshapes of the individual vibronic transitions to a Lorentzian line profile. Justification for the use of a Lorentzian profile is provided by the observation that the helium environment induces fast vibrational relaxation, [52][53][54][55][56] which leads to lifetime broadening of the transitions. To ensure that the resonances were not affected by power broadening, the laser power dependence of each resonance was measured separately.…”

“…10,61 Studies of other molecular systems have revealed a similar difference between in-and out-of-plane vibrational motions, in terms of both line position and linewidth. 52,53 Apparently, the interaction between the molecule and the surrounding helium environment exhibits a considerable anisotropy for nuclear movement along the different vibrational coordinates. The linewidths of the other transitions in the spectrum of 2-aminopurine follow the general trends as well.…”

A new sensitive detection scheme for helium nanodroplet isolation spectroscopy: application to benzene

“…Whereas vibrations in electronically excited states relax just as efficiently as in the electronic ground state, the electronic excitation itself relaxes much slower. 15 This can have a profound effect on the amount of energy deposited into the droplet, especially if the molecule has a large fluorescence quantum yield. Following fluorescence, the vibrational energy content of the molecule is in general substantially lower than the photon energy.…”