We report new measurements of the two-photon absorption rate in the B←X(0,0) 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 <i>Q</i> branch near 230 nm with a nonlinear model for the pulse attenuation, we derive the spectrally integrated cross section σ<sub>0</sub><sup>(2)</sup>. The space- and time-dependent model considers two-photon absorption followed by one-photon photoionization of the excited state. The <i>Q</i>-branch spectral profile is simulated with previously measured coefficients of collisional broadening and shift. Remarkably, for modest irradiances of 80–350 MW/cm<sup>2</sup> at the focus, attenuations of 10% and greater at the <i>Q</i>-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 σ<sub>0</sub><sup>(2)</sup>=(1.5+0.7/−0.2)×10<sup>−35</sup> cm<sup>4</sup>.
© 1999 Optical Society of America
(300.1030) Spectroscopy : Absorption
(300.6390) Spectroscopy : Spectroscopy, molecular
(300.6410) Spectroscopy : Spectroscopy, multiphoton
(300.6420) Spectroscopy : Spectroscopy, nonlinear
(300.6540) Spectroscopy : Spectroscopy, ultraviolet
Michael D. Di Rosa and Roger L. Farrow, "Two-photon excitation cross section of the B ← X(0,0) band of CO measured by direct absorption," J. Opt. Soc. Am. B 16, 1988-1994 (1999)