Abstract

A longstanding goal of laser chemistry in general, and of molecular control in particular, is selective scission of chemical bonds by laser light. Early attempts employing nanosecond IR laser pulses were hindered because the excitation time exceeded the timescale of intramolecular vibrational redistribution (IVR), precluding selectivity. With the advent of femtosecond infrared laser technology, however, novel concepts have been developed. Specifically, Chelkowski et al. [1] have proposed the use of properly chirped laser pulses to enhance vibrational excitation and dissociation. The concept is intuitively clear assuming a multiphoton ladder climbing progression up the vibrational manifold. A negatively chirped IR laser pulse should enhance excitation to higher levels in an anharmonic potential, since the level spacing decreases with increasing energy. Applying a positively chirped IR laser pulse should conversely inhibit excitation. Although not yet realized for molecular dissociation, this concept has been applied to control vibrational population transfer in gas-phase NO [2] as well as solution-phase W(CO)₆ [3]. Recently, it was shown that even without any chirp, metal-carbonyls could be dissociated in the gas phase with intense mid-infrared fs laser pulses tuned to the C-O stretch resonance at 5µm (Fig. 1), which requires an excitation to at least v=7 [4]. We now go a step further and investigate the role of chirp on the dissociation yield of Cr(CO)₆.

© 2002 Optical Society of America