Communication: Tunnelling splitting in the phosphine molecule

Abstract

Splitting due to tunnelling via the potential energy barrier has played a significant role in the study of molecular spectra since the early days of spectroscopy. The observation of the ammonia doublet led to attempts to find a phosphine analogous, but these have so far failed due to its considerably higher barrier. Full dimensional, variational nuclear motion calculations are used to predict splittings as a function of excitation energy. Simulated spectra suggest that such splittings should be observable in the near infrared via overtones of the ν2 bending mode starting with 4ν2.

Phosphine gas in the cloud decks of Venus

“…We take PH 3 1-0 line broadening coefficients to range from 0.186 cm −1 atm −1 , (our theoretical estimate) to 0.286 cm −1 atm −1 (the measured value for the CO 2 broadening of the NH 3 1-0 line). Ammonia and PH 3 share many similarities (see ' Abundance retrieval' in Methods), and can be expected to have comparable broadening properties 30,31 . With this range of coefficients, derived abundances range from ~20 ppb (using our theoretical estimate) up to ~30 ppb (using the proxy NH 3 broadening).…”

Upper and lower bounds for tunneling splittings in a symmetric double-well potential

“…[1][2][3][4][5] It is of interest in quantum eld theory, 6,7 statistical physics, 8,9 and molecular dynamics. 10,11 Recent molecular examples, and this is by no means a comprehensive list, include tunneling splittings in ammonia and phosphine, 12 malonaldehyde, 13,14 and ring-puckering vibrations. 15 The theoretical study of tunneling splittings in molecular systems has a long history.…”

“…For example, a calculation of tunneling splitting in phosphine was performed by exploiting symmetries in ref. 12. Malonaldehyde was studied using the Lanczos construct.…”

A variationally computed room temperature line list for AsH3

“…Of this, the predicted value of 1.457 Å by Schwerdtfeger et al is in reasonable agreement with our own (see Table 3). For comparison, the NH 3 barrier height is measured to be 1786.8 cm −1 occurring for r SP = 0.99460 Å 67 , and for PH 3 the calculated values of Sousa-Silva et al 39 are currently the most reliable, predicting a value of 11 130 cm −1 at 1.3611 Å. Whereas the NH 3 inversion splitting is well known to be ≈ 0.79 cm −1 for the ground vibrational state, it has been predicted but not observed in PH 3 39,68 and so it is unlikely to be observed in AsH 3 for some time.…”