Apparent Isotope Shifts Observed by Two-Step Optogalvanic Spectroscopy

Abstract

Optogalvanic spectroscopy (OGS) is a method for efficiently detecting atomic and molecular spectra. This technique has been used in frequency locking and stabilization of lasers. So far, various state-selective spectroscopies have been reported. In this note, we suggest that the isotope shifts observed in two-step OGS are affected by the incident direction of exciting lasers, and we estimate the isotope shift of the 1s2–2p4 transition of neon. The schematic energy diagram of a neon atom is shown in Fig. 1. We used a tunable single mode laser diode (LD; Newport, 2010M) with a spectral width of 4MHz, resonant with 1s2–2p4 transition, and a multimode (consisting of 3 to 5 lines) He–Ne laser (Melles Griot, LHP925) with a longitudinal mode spacing of 257MHz, resonant with 2p4–3s2 transition. As shown in Fig. 2, the two laser beams were crossed at an angle of less than 10 and were counterpropagated (case A) and copropagated (case B). A commercial hollow cathode lamp (Hamamatsu Photonics, L2783-3NE-LI) with a Ne pressure of 800 Pa was dc discharged. The discharge voltage and laser power were set to values that prevented self-absorption and self-reversal. The He–Ne laser was chopped at a frequency of f (typically 330Hz), and the two-step excitation signal was detected by a lock-in amplifier. Figure 3 shows the OGS signals obtained without a He–Ne laser (we call it conventional OGS in this note) and detected by scanning the LD frequency through the 1s2–2p4 transition of Ne, along with two-step OGS signals in cases A and B. The conventional OGS signal (1s2–2p4) is relevant to the stationary state of Ne, and the line is Doppler broadened. The isotopic component of Ne appears at the right foot of that of Ne. The resolution in two-step OGS is higher than that in conventional OGS. The central frequency of the 633 nm line (2p4–3s2 transition) of Ne is higher than that of Ne, and this shift is denoted by II. This value is reported to be 0.9GHz by Stahlberg et al. by considering that the He–Ne laser line consists of the lines only from the isotope Ne. In both cases A and B in Fig. 2, the velocity components of Ne atoms are toward the He–Ne laser in order to be Doppler shifted by II and resonant with the 2p4–3s2 transition of Ne atoms. In case A, such Ne atoms are away from the LD and the frequency of the LD should be higher (blue shifted) than the central frequency of the 1s2– 2p4 transition by II. On the other hand, in case B, the two laser beams are in the same direction and the frequency of the LD should be red shifted by II. The higher resolution seen in the two-step excitation is explained by such velocity selection. The apparent isotope shift is determined by deconvoluting the observed curve with two Gaussian peaks, as shown for case B in Fig. 3. The isotope shift for the 1s2–2p4 transition should be the mean value of the two apparent shifts obtained by the two schemes of two-step OGS. The isotope shift thus obtained is estimated at 1:8 0:1GHz. If a He–Ne laser with a single mode is used, the peaks of two-step OGS will become much sharper and a more accurate value of the isotope shift will be obtained.