DOPPLER-FREE TWO-PHOTON EXCITATION OF THE VACUUM ULTRAVIOLET ABSORPTION SPECTRUM OF CO

Author Institution: Department of Chemistry, The Pennsylvania State UniversityDoppler-free excitation of the $A^{1}\Pi-X^{1}\Sigma$ transition in CO was produced by two-photon absorption from two counter-propagating beams of ultraviolet radiation. Visible radiation was produced by a Molectron DL-300 dye laser pumped with about 25\% of the output beam of a Molectron UV-1O0O $N_{2}$ laser. The dye laser output of typically 0.05 mJ pulses of $0.03\;cm^{-1}$ bandwidth was amplified by a second dye cell pumped with the remaining 75\% of the $N_{2}$ laser output. The dye amplifier output of 0.5 mJ pulses was frequency doubled with an angle tuned KDP crystal and focussed into a cell containing 5 Torr of CO end reflected back to the same focal point with a curved mirror. Observation of the vacuum ultraviolet fluorescence was used to detect the excitation spectrum. An error voltage derived from the position and intensity of the ultraviolet beam before it was focussed into the CO cell was used to drive the angular orientation of the KDP crystal with a servo motor. In this way the beam position is kept constant, and the UV intensity kept at a maximum during a wavelength scan of 40 {\AA} in the visible without making manual adjustments. CO spectra $(A^{1}\Pi-X^{1}\Sigma_{2_{1}}v^{\prime}= 0\; to\; v^{\prime}=9)$ were observed covering the region $65000\;cm^{-1}$ to $77000\;cm^{-1}$. The bands show O and S branches in addition to the P, Q and R. The vacuum UV spectral Linewidths of $0.12\;cm^{-1}$ are below the Doppler width and are currently limited by the dye laser linewidth. Part of the visible laser radiation was used to excite the spectrum of iodine which was displayed simultaneously with the CO spectrum. Since an atlas of the $I_{2}$ spectrum is available with some lines known to $0.001\;cm^{-1}$, a rather good wavelength calibration is possible