New bands of deuterated nitrous acid (DONO) in the near-infrared using FT-IBBCEAS

The first measurements of near-infrared bands of deuterated nitrous acid (DONO) are presented. The measurements were made using Fourier-Transform Incoherent Broad-Band Cavity-Enhanced Absorption Spectroscopy (FT-IBBCEAS) in the 5800–7800 cm region. Two bands of trans-DONO centred at 6212.029 and 7692.496 cm were observed and assigned to the 2ν1+ν3 combination and 3ν1 overtone vibrations, respectively. Their rotational band structure was satisfactorily reproduced using PGOPHER. For cis-DONO the 3ν1 band was observed at ~7302.5 cm. In addition, new bands centred at 6142.5 cm and 7607.6 cm were quite confidently assigned to the 2ν1+ν3 and 3ν1 vibrations of deuterated nitric acid, DNO3, respectively.


Introduction
Nitrous acid, HONO, has been the subject of numerous spectroscopic studies [1][2][3][4][5][6][7][8][9][10][11][12]. One of the main reasons for studying this species and in its deuterated counterpart DONO [13,14] is its important role in atmospheric photochemistry. As the main precursor of the hydroxyl (OH) radical in polluted urban areas [15,16], HONO plays a significant role in the formation of ozone in the troposphere. The hydrolysis of NO 2 on heterogeneous surfaces is the proposed mechanism for formation of nitrous acid, resulting also in HNO 3 as by-product [17][18][19]. Atmospheric and laboratory measurements of HONO are essential to understand its molecular and photochemical properties as well as the processes of its formation. The near-infrared region is interesting in these aspects because it might play a role in photodissociation from vibrationally excited ("solar pumped") states [20], and also because it is well suited for fast HONO detection in chemical reactions [21].
HONO and DONO are planar molecules with C s symmetry, having six vibrational fundamental modes and two isomeric forms [22,23]. The trans-isomer is the thermodynamically preferred configuration in both cases, as determined by measurements in the microwave region [24]. The energy difference between the ground vibrational states of transand cis-HONO was re-investigated by Sironneau et al. [25] by considering the relative absorption line intensities in the far infrared, which were used to determine the zero-point and ground state energy differences of trans-and cis-HONO and -DONO. The zero-point corrected ground state energies of the cis-isomers of HONO and DONO liẽ 107 cm −1 and~114 cm −1 above that of the corresponding trans-form [23]. Therefore the ground states of the cis-isomers of both isotopologues are thermally populated at room temperature (296 K) at somewhat less than 60% of that of the trans-form.
In contrast to HONO, there have been much fewer studies on DONO [12][13][14]. In fact, no absorption band of DONO has been measured upto-date in the near-infrared region. High-resolution Fourier-transform spectra of DONO were first recorded by Halonen et al. [13], who analysed the ν 3 , ν 4 , ν 5 and ν 6 fundamental bands of trans-DONO and the ν 4 fundamental of cis-DONO. The ν 3 band of trans-DONO was observed to https://doi.org/10.1016/j.cpletx.2020.100050 Received 15 March 2020; Received in revised form 27 April 2020; Accepted 4 May 2020 be perturbed by the nearby ν 5 +ν 6 vibrational state [13]. The ground state constants published in this work were later revised by Dehayem-Kamadjeu et al. [12] using pure rotational spectra in the far-infrared region between 40 and 150 cm −1 . Detailed studies of the ν 1 fundamental band of trans-and cis-DONO between 2350 and 3000 cm −1 measured at 0.003 cm −1 resolution, were also carried out by the same group [14]. Unlike the ν 1 band of HONO, no strong perturbations were observed for either isomer of DONO. The trans-isomer showed very small perturbation for the Q-branch, shifting some lines by up to 0.01 cm −1 , while the ν 1 band of cis-DONO showed no rotational perturbation at all. This was attributed to the distribution of vibrational energy levels, supported by coupled-cluster ab-initio calculations [26], which predict the nearest states to ν 1 to be at least 50 cm −1 away in energy.
In this work, we report measurements of the 2ν 1 +ν 3 and the 3ν 1 bands of trans-DONO and the 3ν 1 band of cis-DONO in the near-infrared region using Fourier-Transform Incoherent Broad-Band Cavity-Enhanced Absorption Spectroscopy (FT-IBBCEAS) [27][28][29]. All bands were sufficiently resolved to yield spectroscopic constants with reasonable accuracy. In addition, the first observation of the 2ν 1 +ν 4 and the 3ν 1 (O-D stretch) bands of deuterated nitric acid (DNO 3 ) is reported. Several bands of HONO, HNO 3 and NO 2 have been detected simultaneously over the full spectral measurement range (5800-7800 cm −1 ), demonstrating the potential of this method to detect different species which might be of interest for future laboratory studies of nitrogen oxides and their related acids.

Experimental
All spectra were recorded at a resolution of 0.08 cm −1 between 5800 and 7800 cm −1 using FT-IBBCEAS [27,28]. IBBCEAS measures the intensity of light transmitted by a stable optical cavity consisting of high reflectivity mirrors (typically R > 99.9%) [29]. The transmission signal strength is measured with and without the absorber of interest present inside the cavity (I λ ( ) and I λ ( ) 0 respectively). From the ratio of the wavelength-dependent transmitted intensities, the reflectivity of the mirrorsR λ ( ), and the sample path length per pass d inside the cavity, the sample's extinction coefficient ε λ ( ) is calculated as . Broadband spectroscopy allows a significant spectral range to be covered simultaneously. This multiplex advantage enables several species to be monitored at once, while gathering information over the entire spectral region [30]. A schematic sketch of the experimental setup is shown in Fig. 1 (also see Ref. [31]). The FT-IBBCEAS set-up employs a supercontinuum fibre laser (Fianium, SC450,~5 W) which has a spectral output from about 5500 to 20000 cm −1 . It runs at a repetition rate of 60 MHz delivering pulses of about 5 ps duration. The optical cavity of 644 cm physical length consisted of two high reflectivity mirrors (R~0.999 from 5714 to 8000 cm −1 , diameter 40 mm, Layertec GmbH). For measurements in the region of interest, a long-pass filter with cut-on wavelength at~9090 cm −1 (Thorlabs, FEL1100nm) was introduced between the laser and the cavity to avoid too much optical power reaching the detector outside the high reflectivity range of the mirrors. The light exiting the cavity was coupled into a Fourier-transform spectrometer (Bruker, Vertex 80), and the transmitted intensity after the cavity was typically integrated for 10 min by averaging over 20 individual interferograms, which were stablished through continuous scanning. The mirror reflectivity was determined by filling the chamber with a known concentration of CO 2 (pressure 6 mbar), and estimating the extinction coefficient based on the absorption cross-section of the sample at the wavenumbers where CO 2 absorption lines are available. From our measurements, we estimate the reflectivity to be 0.9986 at 6050 cm −1 , where the 2ν 3 +ν 1 band of trans-DONO is located; this corresponds to an effective pathlength of~4.6 km in that wavenumber region.

Results and discussion
As a first test of the FT-IBBCEAS set-up, simultaneous measurements of HONO, HNO 3 and NO 2 were carried out by introducing 3.6 mbar NO 2 into the cavity. The NO 2 reacted with the residual water vapour on the cavity walls, forming HONO and HNO 3 . An overview spectrum across a broad spectral range spanning > 2000 cm −1 was measured, from which simultaneous information on several bands of HONO, NO 2 and HNO 3 was obtained. Fig. 2 presents a portion of the FT-IBBCEAS spectrum which was obtained using an acquisition time of 5 min at a resolution of 0.08 cm −1 . For NO 2 the lower limit of the absorption cross section was estimated for the strongest line in the ν 1 +3ν 3 band at 5994.4 cm −1 . A value of~3.1 × 10 −22 cm 2 /molecule was derived from the calibrated reflectivity in that region and the initial pressure of NO 2 . Since NO 2 losses occur in the generation of HONO and HNO 3 , the above value is merely a lower limit. To produce a sample of DONO, the cavity was first passivated overnight with D 2 O, following measurements of a mixture of 6.5 mbar of D 2 O and 4 mbar of NO 2 . The reaction of NO 2 with D 2 O produced trans-as well as cis-DONO. Also overtone bands of DNO 3 were detected for the first time as a result of its formation as by-product of the reaction between NO 2 and D 2 O. The FT-IBBCEAS measurements showed one new vibrational overtone of cis-DONO and two new vibrational overtone bands of trans-DONO (Figs. 3-5). In Fig. 3 the spectral region from 7100 to 7750 cm −1 of the deuterated species is shown, which contains the 3ν 1 bands of cis-and trans-DONO.

Cis-DONO
To corroborate the assignment of the band at 7302 cm −1 a line-byline simulation with the same software as used in Ref. [14] was performed and compared with the measured spectrum, shown in the inset of Fig. 3. For the simulation, the rotational and centrifugal distortion constants of the ν 1 band of cis-DONO from Dehayem et al. [14] were used and then extrapolated to estimate those of the 3ν 1 overtone state. The calculation used an S-reduced Watson Hamiltonian in the I r representation [33]. As observed for the ν 1 band [14], a pure b-type spectrum was assumed for the 3ν 1 band. Only the band centre was adjusted to achieve good overall agreement. The similarity of the band shapes in the inset of Fig. 3 indicates that this band is indeed the 3ν 1 band of cis-DONO.

Trans-DONO
Trans-DONO is a near-prolate asymmetric top (κ = -0.95). For the analysis of the rotational structure of the 2ν 1 +ν 3 and 3ν 1 vibrational bands of trans-DONO (shown in Figs. 4 and 5), it was assumed that the   Table 1 for the upper state's rotational constants. A temperature of 300 K was used for the simulation. The Q-branch in the modelled spectrum is sharper, probably due to differences in the centrifugal distortion constants, and possibly also line-mixing. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) The agreement is quite satisfactory despite some noise in the measured spectrum. See Table 1 for the upper state's rotational constants. A temperature of 300 K was used. Note the K-structure of the Q-branch in both the measured and the modelled spectrum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) excited vibrational state has A 1 symmetry. The rotational structure of the upper levels was then modelled with the PGOPHER software [33] using the known ground-state rotational and centrifugal distortion constants [12], which were kept constant in the analysis. A Gaussian linewidth of 0.10 cm −1 and a rotational temperature of 300 K were used for the spectral modelling.
The band centres of 2ν 1 +ν 3 and 3ν 1 were determined to be at 6212.0292 and 7692.4960 cm −1 . The central Q-branches are the most dominant features. No resolved line structure is observed for the 2ν 1 +ν 3 combination band (see Fig. 4), but a beautiful K-substructure is observed for the 3ν 1 overtone band (see Fig. 5). Both bands show distinct a-type features in the P-and R-branches with spacings of about 0.7 cm −1 (about B + C, characteristic for parallel bands of prolate symmetric tops). One can assign a-type q Q K lines up to K a = 12 in the case of trans-3ν 1 (see Fig. 5).
The spectroscopic constants from the PGOPHER analysis of the trans-DONO bands are summarized in Table 1 together with the constants used for modelling the cis-DONO band in the inset of Fig. 3. All quantities in the S-reduced Watson-Hamiltonian (I r representation) that are not shown in the table were fixed to zero in the calculation. This is justified by the much smaller values of d 1 and d 2 for DONO compared to the diagonal centrifugal distortion constants [14].
In Table 2 some known elements of the harmonic progression of the ν 1 vibration are compared for both isomers of HONO and DONO. As expected the anharmonicity of the 3ν 1 for DONO is smaller than that of HONO based on the larger mass of DONO [35]. cis-DONO appears to be somewhat more anharmonic than trans-DONO (by~38 cm −1 ), which follows the same trend as for the 2ν 1 of cis-HONO and trans-HONO, where again cis-HONO shows the larger anharmonicity (~24 cm −1 ); see Table 2.

DNO 3
In the NO 2 /H 2 O mixture, in addition to nitrous acid HONO, nitric acid, HNO 3 , was observed (see Fig. 2) in the FT-IBBCEA spectrum. Similarly, the FT-IBBCEA spectrum of a NO 2 /D 2 O mixture, apart from the DONO combination overtones discussed above, spectroscopic features due to DNO 3 were observed around 6143 and 7608 cm −1 . Even though it is expected that DNO 3 is formed as a by-product of the heterogeneous reaction of D 2 O and NO 2 (see Fig. 3), it was validated with a sample of pure DNO 3 that the bands centred at 6142.5 cm −1 and 7607.6 cm −1 can be assigned to bands of deuterated nitric acid. It is highly likely that the former represents the 2ν 1 +ν 3 combination band of DNO 3 , while the latter corresponds to the 3ν 1 overtone, as calculated from the mid-infrared bands of DNO 3 [37]. The respective spectral regions are shown in Fig. 6; the rotational structure of both bands is not fully resolved, but sharp Q-branches located at 6142.5 cm −1 and 7607.6 cm −1 dominate the bands in both cases, as previously observed for HNO 3 [17]. Note that a-type bands are expected for these overtone and combination vibrations involving ν 1 also in DNO 3 .

Conclusions
In this study, the first observation of combination and overtone bands 2ν 1 +ν 3 (6212.0 cm −1 ) and 3ν 1 (7691.8 cm −1 ) of trans-DONO and of the 3ν 1 (~7302.5 cm −1 ) of cis-DONO in the near-infrared is reported. The spectra were recorded using FT-IBBCEAS and show the high suitability of this method for measurements of isotopic trace species. In addition, due to the formation of deuterated nitric acid in the same experiment, two new bands of DNO 3 at 6142.5 cm −1 and 7607.6 cm −1 were observed, which are very likely assigned to the 2ν 1 +ν 3 and 3ν 1 respectively. Follow-up high resolution measurements of these isotopologues as well as high-end theoretical calculations of this interesting region are highly desirable.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Table 1 Spectroscopic constants (in cm −1 ) of the 2ν 1 +ν 3 and 3ν 1 bands of trans-DONO and of the 3ν 1 band of cis-DONO. The S-reduced Watson-Hamiltonian in the I r representation was used [33]. (a) Values obtained in this study from a fit using PGOPHER [34]. Numbers in parentheses are 1σ uncertainties in units of the last digit.  [14]. (c) Vibrational dependence of rotational and centrifugal distortion constants from Ref. [14] extrapolated to three quanta of ν 1 , only the band centre was adjusted to match the simulated with the observed data.