To achieve large population transfer to high vibrational levels in a selected ground-state mode of a polyatomic molecule [Cr(CO)6], we apply chirped femtosecond mid-infrared laser pulses at 2000 cm−1 to optimize vibrational ladder climbing as an energy deposition mechanism, which in turn controls the outcome of a unimolecular dissociation process. Its dependence on excitation parameters (frequency, intensity, chirp) is investigated and found to be in excellent agreement with a theoretical calculation. In particular, it is shown that optimizing vibrational ladder climbing allows for coherently controlled excitation even in a polyatomic molecule.
Gaseous diazomethane (CH2N2) has been irradiated with femtosecond laser pulses tuned to the CNN asymmetric stretch at 2100 cm−1 in the mid-infrared. Laser-induced fluorescence detection of CH21 [537 nm, b̃1B1(0 16 0)←ã1A1(0 0 0)] confirms infrared (IR) multiphoton-induced scission of the C–N bond on two distinct time scales, 480±70 fs and 36±8 ps. The longer time scale is consistent with a statistical dissociation process; the shorter one is indicative of directed dissociation, which occurs more rapidly than statistical intramolecular vibrational energy redistribution because of direct coupling of the vibrational energy from the IR-excitation mode into the reaction coordinate. Anisotropy measurements demonstrate that the CH2 group bends significantly out of the molecular plane prior to the dissociation.
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.
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