The gas phase vibrational overtone spectrum of propane is measured using conventional near infrared (NIR) spectroscopy for the ΔvCH=2–5 regions and intracavity dye laser photoacoustic spectroscopy (IDL-PAS) for the ΔvCH=5 and 6 regions. The peaks are assigned in terms of the local mode model. Experimental oscillator strengths are compared to values calculated for the CH-stretching components of the spectrum. The calculations use a harmonically coupled, anharmonic oscillator local mode model to obtain the vibrational wave functions, and ab initio MO calculations at the SCF level with a 6-31G* basis set to obtain the dipole moment function. The importance of intermanifold coupling is explored. The calculations can account for the fall-off in intensity with increasing v, and can give a reasonably quantitative account of the relative intensities of the individual peaks within a given vibrational manifold. The questions of the relative intensities of primary and secondary CH bonds, and of the relative intensities of different methyl CH bonds are also explored.

Numerical simulations of high-order harmonic generation in the molecular ions H 2 ϩ and H 3 2ϩ show that two-color intense (IӍ10 14 W/cm 2 ) short (Ͻ1-psec) laser pulse excitation produces harmonics at large internuclear distances with larger efficiencies ͑intensities͒ at 6U p , 8U p , and 10U p energies than in single-atom single-frequency excitation. The two-color excitation scheme creates efficient high harmonics by first ionizing electrons with a high-frequency laser pulse and accelerating them with lower frequency towards neighboring atoms. Such two-color schemes should be useful for enhancing high-order harmonic generation in clusters.

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