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A Calibration system for the visible region with application to the NO₂ and BO₂ molecules

University of British Columbia

A method for calibrating high resolution laser spectra in the visible region has been developed and tested by taking ultra-high resolution molecular-beam spectra of NO₂ and sub-Doppler intermodulated fluorescence spectra of BO₂. The calibration system is based on an evacuated Fabry-Perot etalon whose cavity length is servo-locked to a stabilized HeNe laser; the absolute order number of the fringe at the HeNe frequency is known and the free spectral range can be determined with high accuracy. For absolute frequency measurements, the order number of a transmission fringe is obtained from a commercial wavemeter (whose accuracy is sufficient to identify the fringe); the absolute frequency is then the HeNe frequency multiplied by the ratio of the unknown' order number to the 'lock point' order number. Two K=0 subbands of the ²B₂ - ²A₁ electronic transition of NO₂ at 593.5 nm and 585.1 nm were chosen for analysis. The hyperfine transitions associated with rotational levels up to N"=10 were studied; although the system is highly perturbed, it was possible to obtain a series of meaningful constants describing the excited state. Values for the electron spin-rotation, Fermi contact and (I,S) dipolar parameters were obtained. Good agreement was found with literature values where they exist. Using the calibration system the small frequency splittings of the hyperfine intervals were measured accurate to ±1 MHz, as determined by comparison with earlier microwave work. Larger frequency intervals (between rotational levels) were found to be consistent to better than 10 MHz, thereby demonstrating the accuracy of the calibration system over small (a few MHz) and somewhat larger (~480 GHz) frequency ranges. The rotational analysis of BO₂ covered the region near 5400Å which allowed the (0,0) and 2[sup 1/sub 1] bands of the A²π[sub u] and X²π[sub g] transitions to be analyzed. The ground state constants obtained agreed well with previous results while the A²π[sub u] upper state exhibited two very interesting features: there is a K-resonance avoided crossing between the 010 ²Δ[sub 5/2] and ²Σ⁺ vibronic levels, while the ²Δ[sub 3/2] level lies entirely between the F₁ and F₂ spin components of the ²Σ⁻ vibronic level. These features have permitted the relative energies of all the vibronic levels of the 010 vibrational levels to be determined accurately in both electronic states. This is not normally possible in ²π -²π electronic transition. Both the v₂=0 and v₂=1 upper states are randomly perturbed by high lying ground state vibronic levels; this is an unexpected effect for such comparatively low energies, and would have gone unnoticed but for the extremely high precision of the data. Once again the calibration system proved its power, with rotational combination differences in bands with a common upper state, separated by more than 200 cm⁻¹, being reproduced, on average, to ± 0.0003 cm⁻¹.

Text

2010-08-23

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http://hdl.handle.net/2429/27639

Chemistry

University of British Columbia

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