Intramolecular quantum chaos in doped helium nanodroplets E. Polyakova a , D. Stolyarov a , X. Zhang a , V.V. Kresin b , C. Wittig a, * a Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA b Department of Physics, University of Southern California, Los Angeles, CA 90089, USA Received 4 October 2002; in final form 8 April 2003 Published online 16 June 2003 Abstract A mass spectrometric depletion spectrum (17700–18300 cm 1 ) is reported for NO 2 in superfluid (0.37 K) helium nanodroplets. Gas phase NO 2 is believed to be vibronically chaotic at these energies. Transitions are broadened and blue-shifted relative to their gas phase counterparts. The spectrum is fitted reasonably well by setting all of the widths and shifts equal to 7 cm 1 . Modest dispersions (i.e., 90% lie within 2 cm 1 of the central values) are consistent with quantum chaos in NO 2 . Relaxation is dominated by interactions of NO 2 with its non-superfluid helium nearest neighbors. Ó 2003 Elsevier Science B.V. All rights reserved. 1. Introduction Superfluid helium nanodroplets (hereafter referred to as He n , where n is the number of He atoms) provide convenient environments for car- rying out detailed studies of embedded molecules and aggregates under well-characterized, ultracold (0.37 K) conditions [1–3]. Research in this area has flourished following the seminal experiments of Vilesov, Toennies, and coworkers, in which mo- lecular-level manifestations of He n superfluidity were revealed in a series of elegant spectroscopic studies [4–7], and several exciting directions have been identified [8–10]. At this point in time, a significant number of fundamental and overtone vibrations of molecules and weakly bound complexes in their ground po- tential energy surfaces (PESs) have been examined [11–13]. In addition, molecular electronic spectra have revealed phonon wings, including a Ôroton gapÕ, radiationless decay, and details of the dop- ant–He n interactions [14–18]. These studies have been carried out mainly in the regime of regular nuclear dynamics, where the goodness of vibra- tional quantum numbers is high. On the other hand, vibrational congestion, which often goes hand-in-hand with complex dynamics, has been noted in electronic spectra of large molecules, e.g., tetracene and pentacene [15,16]. However, in sys- tems of such high dimensionality (i.e., having 84 and 108 vibrational degrees of freedom, respec- tively), state-resolved studies in the regime of chaotic dynamics are not feasible. This article reports the first spectroscopic ob- servations – in a regime where the intramolecular Chemical Physics Letters 375 (2003) 253–260 www.elsevier.com/locate/cplett * Corresponding author. Fax: 1-213-746-4945. E-mail address: wittig@usc.edu (C. Wittig). 0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00714-0