The evidence of the mode selectivity of the infrared predissociation reaction of the hydrogen bonds in the aniline±water±pyrrole cluster cation Taisuke Nakanaga * , Naveed K. Piracha, Fumiyuki Ito Photoreaction Control Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba-Central 5, Higashi 1-1, Tsukuba, Ibaraki 305-8565, Japan Received 26 June 2001; in ®nal form 26 August 2001 Abstract The infrared spectrum of aniline±water±pyrrole cluster cation has been measured in the 3 lm region. Four strong vibrational bands were observed at 3313, 3515, 3637, and 3721 cm 1 , and the cluster dissociated into fragments in two reaction paths, AWP ! AW P and AWP ! AP W A: aniline, W: water, P: pyrrole) when it absorbed an infrared photon. The branching ratio AW =AP has been determined to be 0:20 0:02 for the NH stretching vi- bration of aniline 3313 cm 1 , hydrogen-bonded to pyrrole) and 0:14 0:03 for the other three bands. This vibrational mode dependence should be due to the fast energy transfer from the NH to the intermolecular vibration through the hydrogen bond. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction Bond selective photo-dissociation has been a subject of general interest to laser chemists since the invention of the laser. There has been, how- ever, only one report of bond selective photo-dis- sociation in the ultraviolet region [1] and no example in the visible and infrared spectral region has been reported for one photon process. The diculty lies in the fast energy relaxation between the excited vibrational levels. In order to control the photochemical reactions, we must excite the molecule into a speci®c excited state where the molecule dissociates into fragments eectively. Since usually this cannot be realized by one pho- ton process, a number of photochemical control methods using multi-photon processes are pro- posed and studied [2±5]. Here we present another attempt to realize the bond selective photo-disso- ciation using the hydrogen bonds in clusters. It is well known that the infrared absorption band of the XH X N, O, etc.) stretching vibra- tion shows a red shift and the band shape becomes broad when a XH bond forms the hydrogen bond with the other molecule [6], and large red shift and broad bandwidth are observed in the isolated clusters [7]. Pine and Laerty [8] showed the vib- rational mode dependence of predissociation line width in HF dimer. Since then, similar observa- tions have been reported in various molecular clusters, also the vibrational mode dependence of the predissociation lifetime of several dimer 12 October 2001 Chemical Physics Letters 346 2001) 407±412 www.elsevier.com/locate/cplett * Corresponding author. Fax: +81-298-61-4521. E-mail address: t.nakanaga@aist.go.jp T. Nakanaga). 0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0009-261401)00989-7