Spectroscopy Exposes Molecular Dynamics

Daniel S. Burgess

Researchers at the National Research Council's Steacie Institute for Molecular Sciences have developed a femtosecond-laser-based method to observe coupled electronic rearrangements and atomic motions in molecular processes. The ultrafast mixing of electronic and vibrational motions is a fundamental aspect of photochemistry and photobiology that also underlies molecular electronics.

Albert Stolow, a senior research officer at the institute and project leader, applied time-resolved photoelectron spectroscopy to study the dynamics in a linear hydrocarbon chain. The method employs a femtosecond laser like a stroboscope. The pump laser excites a molecule to an optically bright state, from which it undergoes a rapid redistribution of charge and energy. A probe pulse ionizes the molecule as a function of time as it shifts to a lower, but vibrationally hotter, electronic state. The number of emitted photoelectrons and their kinetic energy spectra are interpreted to determine how the molecule simultaneously redistributes both the charge and energy.

Researchers at the Steacie Institute for Molecular Sciences used femtosecond lasers to observe the ultrafast mixing of electronic and vibrational motions in a linear hydrocarbon chain.

The researchers illuminated all-trans 2,4,6,8 decatetraene (C₁₀H₁₄) with an excitation pulse at 287 nm. The electronically excited -- but vibrationally cold -- molecule produced electrons with a peak kinetic energy of 2.5 eV when ionized at 235 nm. Within 400 fs, the molecule evolved to a 0.7-eV electronic state, but one with vibrational energy. The photoelectron spectra displayed a structure that the team confirmed with the results of a two-photon probe ionization of decatetraene by a 352-nm pulse.

The group used a solid-state pumped femtosecond Ti:sapphire oscillator from Spectra-Physics Lasers Inc. of Mountain View, Calif., and a diode-pumped picosecond Nd:YAG from Lightwave Electronics Corp., also of Mountain View, which were both locked to a crystal oscillator. Two femtosecond optical parametric oscillators from Quantronix Corp. in East Setauket, N.Y., were pumped by a 1-kHz Ti:sapphire regenerative amplifier from Positive Light of Los Gatos, Calif., which boosted the synchronized output.

The National Research Council designed a 1-kHz Nd:YVO₄ regenerative amplifier for the Nd:YAG laser that excited the samples and amplified the second harmonic of the Quantronix oscillators. BBO crystals from Super Optronics in Redondo Beach, Calif., were employed for the nonlinear frequency conversion.

Stolow noted that an understanding of the dynamics of molecular systems is crucial to the design of mo-lecular-scale devices. He expects such work will enable the improved design of molecular electronics for information and telecommunications technology.

The results of the study appear in the Sept. 2 issue of Nature.