Precision Measurement of Vibrational Quanta in Tritium Hydride

Saturated absorption measurements of transitions in the (2-0) band of radioactive tritium hydride are performed with the ultrasensitive noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy intracavity absorption technique in the range 1460–1510 nm. The hyperfine structure of rovibrational transitions of tritium hydride, in contrast to that of hydrogen deuteride, exhibits a single isolated hyperfine component, allowing for the accurate determination of hyperfineless rovibrational transition frequencies, resulting in $\mathrm{R}(0)=203396426692(22)\mathrm{kHz}$ and $\mathrm{R}(1)=205380033644(21)\mathrm{kHz}$. This corresponds to an accuracy 3 orders of magnitude better than previous measurements in tritiated hydrogen molecules. Observation of an isolated component in P(1) with reversed signal amplitude contradicts models for line shapes in hydrogen deuteride based on crossover resonances.

Figure 1

Schematic of the tritium-adapted setup. The optical cavity with two reflective mirrors, a piezo stack, and two sealed optical windows, is fully metal-sealed using indium wire seals (orange). The assembly is inside a secondary vacuum for additional stability and safety. Getter G1 is the tritium source to acquire and store the tritium. Getter G2 serves as an additional backing pump for when the experiment is not active. The third valve enabled initial pump-down. PG, pressure gauge.

Figure 2

(a) Recorded spectra of the R(0) line at a total hydrogen pressure of 0.5 Pa (50% HT) for intracavity powers of 80 W and 400 W. The absolute frequency scale is given via $f_0=203396427135\mathrm{kHz}$. In the inset the single isolated hyperfine component is shown with a fit, the width of the blue bar representing the statistical uncertainty. The width of the purple bar, at the location of the purely rovibrational transition, represents the final uncertainty. (b) Hyperfine components as a stick spectrum (strength indicated by dipole moments $d_e$). (c) Level scheme of hyperfine components.

Figure 3

(a) Recorded spectra of the R(1) line at two different powers, with an isolated Lamb dip at the high-frequency side covering three hyperfine components. The absolute frequency scale is given via $f_0=205380034083\mathrm{kHz}$, corresponding to the fitted center position of the isolated Lamb dip as shown in the inset. Its width (blue bar) represents the sum of the statistical uncertainty and the uncertainty contribution of the three components (see text). The purple bar represents the location and final uncertainty of the purely rovibrational transition frequency. (b) Hyperfine stick spectrum for this transition. (c) Level scheme of hyperfine components; the numbers in between levels denote the level splittings in units of kHz.

Figure 4

(a) Recorded saturated absorption spectra of the P(1) line in the (2-0) band of HT for an intracavity circulating power of 400 W and hydrogen pressures as indicated. The absolute frequency scale is given via $f_0=198824820185\mathrm{kHz}$. (b) Stick spectrum of hyperfine components. (c) Level scheme of hyperfine components.