Appl Phys B (2012) 106:921–926 DOI 10.1007/s00340-011-4811-x Direct determination of the dissociation probability in highly focused IR multiple photon dissociation N.D. Gómez · V. D’Accurso · J. Codnia · F.A. Manzano · M.L. Azcárate Received: 8 July 2011 / Revised version: 3 October 2011 / Published online: 26 November 2011 © Springer-Verlag 2011 Abstract Laser-induced fluorescence (LIF) has been used to directly determine the dissociation probability per pulse in highly focused infrared multiple-photon dissociation (IRMPD). The fluence dependence of CDCl 3 IRMPD has been determined by LIF and FTIR spectrometry. The partic- ular LIF irradiation and detection system’s geometry imple- mented allowed us to monitor the local CCl 2 radicals con- centration in the intersection zone of the observation and the dissociation volumes. The fluence dependence of the LIF in- tensity was modeled with the cumulative log-normal distri- bution. The dependence of the global values of the fraction of molecules dissociated per pulse on fluence was obtained from FTIR spectrometry. The dissociation probabilities per pulse were derived from the deconvolution of these values using the cumulative log-normal distribution. A very good agreement between the values of the parameters σ and Φ sat obtained from the deconvolution technique with those re- Carrera del Investigador de CONICET. N.D. Gómez · V. D’Accurso · J. Codnia · F.A. Manzano · M.L. Azcárate ( ) Centro de Investigaciones en Láseres y Aplicaciones (CEILAP), CITEDEF-CONICET, J.B. de La Salle 4397 (B160ALO), Villa Martelli, Argentina e-mail: lazcarate@citedef.gob.ar Fax: 54-11-47098100 N.D. Gómez e-mail: ndgomez@citedef.gob.ar V. D’Accurso e-mail: vdacurso@citedef.gob.ar J. Codnia e-mail: jcodnia@citedef.gob.ar F.A. Manzano e-mail: fmanzano@citedef.gob.ar sulting from the fluorescence intensity fit was found, show- ing the validity of the method proposed. 1 Introduction Infrared multiple-photon dissociation (IRMPD) is a widely spread technique used in vibrational photochemistry stud- ies and laser isotope separation [1–3]. An important param- eter is the dissociation probability which depends on the laser energy per unit area or fluence. Small and medium size molecules are often characterized by high dissociation en- ergy thresholds. Therefore, focused laser beams are usually employed to obtain high fluences to attain dissociation. As a consequence, there is a strong variation of the fluence in the photolysis volume. Given that in highly focused geome- tries the effective volumes are very small, usually a large number of irradiation pulses is required to obtain appre- ciable dissociation yields. With low repetition rate lasers, the experiments last several hours. The global dissociation yield can be described in terms of an effective volume since the fraction of molecules dissociated per pulse in the irra- diated volume is diluted in the whole cell volume. Global values of the dissociation yield can be obtained from in- frared spectrometry. In addition, deconvolution procedures must be used to obtain the fluence dependence of the disso- ciation probability. Several deconvolution models have been proposed [4–12]. However, a two-parameter model based on the cumulative log-normal distribution has demonstrated to satisfactorily describe the dissociation probabilities of molecules over a wide fluence range in IRMPD [10–12]. In view of these considerations, a technique that would per- mit the direct determination of the dissociation probability in focused geometries would be very advantageous and time saving.