The fundamental ${\nu }_3$ band of DTO and the $2{\nu }_1$ overtone band of HTO from the analysis of a high-resolution spectrum of tritiated water vapour

Highlights

• High-resolution spectrum of tritiated water species further analysed.

• Line positions determined at an accuracy of 10⁻² cm⁻¹ up to a few 10⁻⁴ cm⁻¹.

• Line-by-line analysis of 436 lines for ${\nu }_3$ of DT₁₆O and 361 lines for $2{\nu }_1$ of HT₁₆O.

• Differences to ab initio line positions of the order of 0.1 cm⁻¹.

• Spectroscopic constants determined in A-reduced Watson Hamiltonian.

Abstract

The ${\nu }_3$ fundamental band of DTO and the $2{\nu }_1$ overtone band of HTO have been analysed for the first time from a high-resolution infrared spectrum of tritiated water vapour recorded at room temperature using a Bruker IFS 125HR Fourier transform spectrometer applying a resolution of $0.0075{\mathrm{cm}}^{-1}$.

For DT¹⁶O, we report 436 new experimental line positions in the 2500–$2900{\mathrm{cm}}^{-1}$ range that were assigned to the ${\nu }_3$ band, with rotational quantum numbers up to $J=15$ and $K_a=8$. For HT¹⁶O, we report 361 new experimental line positions in the 4300–$4700{\mathrm{cm}}^{-1}$ range that were assigned to the $2{\nu }_1$ band, with rotational quantum numbers up to $J=15$ and $K_a=7$.

For both species the assigned lines were used to determine the spectroscopic constants in the A-reduced Watson Hamiltonian. No perturbations were observed.

The comparison with variational calculations of line positions and intensities from the Tomsk database shows differences of up to several $0.1{\mathrm{cm}}^{-1}$ concerning the line positions, while the relative line intensities agree with the observed values within a few percent.

The line list from this study will be useful for detection of gaseous traces of HTO using laser-based methods in the future and further serve the completion of experimental line lists.

Introduction

Recently, we have published the first measurement and analysis of the $2{\nu }_2$ band of HT¹⁶O [1]. A historical overview on previous works on tritiated water spectroscopy is given therein. Adding to our earlier results, we here present 436 lines of the fundamental ${\nu }_3$ mode of DT¹⁶O as well as 361 lines belonging to the $2{\nu }_1$ band of HT¹⁶O. Using these positions, we have determined the corresponding spectroscopic constants in the A-reduced Watson formulation for both bands. As early as 1956 the band centre of the ${\nu }_3$ band of DTO had been stated by Staats et al. [2]. Their result of 2735(5) cm⁻¹ for this band centre readily agrees with ours of 2737.39556(44) cm⁻¹. Spectroscopic constants of the ground state of DTO were determined by Helminger et al. [3], which we used when fitting the upper state’s constants. To our knowledge, there are no further high-resolution spectroscopic studies of DTO. For HTO however, the band centres of the fundamental bands are known: ${\nu }_1=2299.7713(56){\mathrm{cm}}^{-1}$ by Cope et al. [4], ${\nu }_2=1332.480(13){\mathrm{cm}}^{-1}$ by Ulenikov et al. [5] and ${\nu }_3=3716.5475(48){\mathrm{cm}}^{-1}$ by Tine et al. [6].

Analogous to our work on the $2{\nu }_2$ overtone band of HT¹⁶O the now experimentally determined line positions may be useful for the refinement of variational calculations, available in databases [7]. Our goal is to provide accurate ro-vibrational energy levels for all tritiated water species over a wide spectral range, which may be useful to achieve spectroscopic accuracy with variational calculations.

Notably isolated from most atmospheric disturbances, the $2{\nu }_1$ overtone band of HT¹⁶O may in particular serve for the detection of tritiated water vapour for current and future applications such as fusion reactor cycles [8]. In this paper we use the standard notation for the vibrational bands of water isotopologues, that is: ${\nu }_1$ is the symmetric stretching, ${\nu }_2$ is the symmetric bending and ${\nu }_3$ is the asymmetric stretching mode.