Abstract
We report new measurements of the two-photon absorption rate in the band of CO, as determined from absorption spectra obtained with Fourier-transform-limited pulses of well-characterized spatial profile. By comparing measured absorption spectra of the CO Q branch near 230 nm with a nonlinear model for the pulse attenuation, we derive the spectrally integrated cross section The space- and time-dependent model considers two-photon absorption followed by one-photon photoionization of the excited state. The Q-branch spectral profile is simulated with previously measured coefficients of collisional broadening and shift. Remarkably, for modest irradiances of 80–350 MW/cm2 at the focus, attenuations of 10% and greater at the Q-branch peak were observed in a single pass through neat CO at pressures of 12–33 kPa. Our result for the spectrally integrated cross section, averaged over five experiments, is
© 1999 Optical Society of America
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