High sensitivity ICLAS of H2O in the region of the second decade (11 520-12 810 cm)

The high resolution absorption spectrum of the H₂¹⁸O isotopologue of water has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) with a sensitivity on the order of α_min ∼ 10⁻⁹ cm⁻¹. The 11 520-12 810 cm⁻¹ spectral region corresponding to the 3ν + δ decade of vibrational states, was explored with an ICLAS spectrometer based on a Ti:Sapphire laser. It allowed detecting transitions with an intensity down to 10⁻²⁷ cm/molecule which is about 100 times lower than the weaker line intensities available in the literature, in particular in the HITRAN database.The rovibrational assignment was performed on the basis of the results of variational calculations and allowed for assigning 3659 lines to the H₂¹⁶O, H₂¹⁸O, H₂¹⁷O, HD¹⁶O and HD¹⁸O species, leaving only 1.7% unassigned transitions. A line list including 1712 transitions of H₂¹⁸O has been generated and assigned leading to the determination of 692 rovibrational energy levels belonging to a total of 16 vibrational states, 386 being newly observed. A deviation on the order of 25% has been evidenced for the average intensity values given by HITRAN and the results of variational calculations. Ninety two transitions of the HD¹⁸O isotopologue could also be assigned and the corresponding upper rovibrational levels are given.     

ICLAS of weak transitions of water between 11 300 and 12 850 cm: Comparison with FTS databases

Very weak water vapor absorption lines have been investigated by intracavity laser absorption spectroscopy (ICLAS) in the 11 335-11 947 and 12 336-12 843 cm⁻¹ spectral regions dominated by the ν₁ + 3ν₂ + ν₃ and ν₂ + 3ν₃ bands, respectively. A detectivity on the order of α_min ∼ 10⁻⁹ cm⁻¹ was achieved with an ICLAS spectrometer based on a Ti: Sapphire laser. It allowed detecting transitions with an intensity down to 5 × 10⁻²⁸ cm/molecule which is about 10 times lower than the weakest line intensities previously detected in the considered region. A line list corresponding to 1281 transitions with intensity lower than 5 × 10⁻²⁶ cm/molecule has been generated. A detailed comparison with the line lists provided by the HITRAN database and by recent investigations by Fourier transform spectroscopy associated with very long multi pass cell is presented. The rovibrational assignment performed on the basis of the ab initio calculations of Schwenke and Partridge, has allowed for determining 176 new energy levels belonging to a total of 16 vibrational states.     

High sensitivity ICLAS of D2O between 12 450 and 12 850 cm

The weak absorption spectrum of dideuterated water, D₂O, has been recorded between 12 450 and 12 850 cm⁻¹ by high sensitivity Intracavity Laser Absorption Spectroscopy (ICLAS). This spectral region corresponds to the (ν₁ + ν₂/2 + ν₃) = 5 polyad, dominated by the 4ν₁ + ν₃ band centered at 12 743.035 cm⁻¹. The achieved sensitivity has allowed for the detection of lines with a minimum intensity of 2 × 10⁻²⁸ cm/molecule i.e. typically two orders of magnitude lower than previous observations in the region considered. A total of 586 energy levels belonging to 11 vibrational states were determined. The rovibrational assignment process of 1025 lines ascribed to D₂O was based on new results of variational calculations by Shirin et al. [S.V. Shirin, N.F. Zobov, O.L. Polyansky, J. Quant. Spectrosc. Radiat. Transfer, in press, doi:10.1016/j.jqsrt.2007.07.010]. The overall agreement between these calculations and the observed spectrum is good both for the line positions and line intensities. The difficulties encountered while performing the rovibrational labeling and the assignment of the weakest transitions not included in Combination Differences relations, are discussed.     

Intracavity laser absorption spectroscopy of D2O between 11 400 and 11 900 cm

The weak absorption spectrum of dideuterated water, D₂O, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) between 11 400 and 11 900 cm⁻¹. This spectrum is dominated by the 3ν₁ + ν₂ + ν₃ and the ν₁ + ν₂ + 3ν₃ centered at 11 500.25 and 11 816.64 cm⁻¹, respectively. A total of 530 energy levels belonging to eight vibrational states were determined. The rovibrational assignment process of the 840 lines attributed to D₂O was mostly based on the results of new variational calculations consisting in a refinement of the potential energy surface of Shirin et al. [J. Chem. Phys., 120 (2004) 206] on the basis of recent experimental observations, and a dipole moment surface from Schwenke and Partridge [J. Chem. Phys. 113 (2000) 6592]. The overall agreement between these calculations and the observed spectrum is very good both for the line positions and the line intensities.     

The absorption spectrum of water between 13 540 and 14 070 cm: ICLAS detection of weak lines and a complete line list

The absorption spectrum of water vapor has been investigated by Intracavity Laser Absorption Spectroscopy (ICLAS) between 13 540 and 14 070 cm⁻¹. This spectrum is dominated by relatively strong transitions of the 4δ polyad of vibrational states. The achieved sensitivity - on the order of α_min ∼ 10⁻⁹ cm⁻¹ - has allowed one to newly measure 222 very weak transitions with intensities down to 5 × 10⁻²⁸ cm/molecule at 296 K. Fifty new or corrected H₂¹⁶O energy levels belonging to a total of 13 vibrational states could be determined from the rovibrational analysis based on variational calculations by Schwenke and Partridge. The previous investigations in the region by Fourier Transform Spectroscopy were critically evaluated and used to construct the best to date set of energy levels accessed by transitions in the considered region. All the rovibrational transitions reaching these upper energy levels and having intensities larger than 4.0 × 10⁻²⁸ cm/mol were calculated. In the resulting line list, the positions at the level of experimental accuracy were augmented with variational intensities leading to the most complete line list for water in normal isotopic abundance in the 13 500-14 100 cm⁻¹ region.     

D2O: ICLAS between 13 600 and 14 020 cm and normal mode labeling of the vibrational states

The high resolution absorption spectrum of dideuterated water, D₂¹⁶O, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) in the 13 600-14 020 cm⁻¹ spectral region which is the highest energy region reported so far for this water isotopologue. Because the HD¹⁶O absorption is stronger by three orders of magnitude in the region under study, it was necessary to use high deuterium enrichment in order to minimize the HD¹⁶O absorption lines overlapping the D₂¹⁶O spectrum. With the high sensitivity achieved (noise equivalent absorption α_min ∼10⁻⁹ cm⁻¹), transitions with line strengths on the order of 5 × 10⁻²⁸ cm molecule⁻¹ could be detected. The spectrum analysis, based on recent variational calculations has provided a set of 177 new rovibrational energy levels belonging to six vibrational states.The most complete set of 53 vibrational energy levels of D₂¹⁶O, including the three newly determined band origins, was constructed from an exhaustive review of the literature data. The fitting of the parameters of the vibrational effective Hamiltonian has allowed to reproduce the whole set of vibrational energies with an rms deviation of 0.055 cm⁻¹. This simple model gave consistent vibrational labels of the D₂¹⁶O states up to 18 000 cm⁻¹. Above 15 000 cm⁻¹, Fermi and Darling-Dennison resonance interaction were found to induce strong vibrational mixings of the wave functions in the normal mode basis, leading to ambiguous vibrational labeling.     

Intracavity Laser Absorption Spectroscopy of D2O between 12 850 and 13 380 cm

The high resolution absorption spectrum of dideuterated water, D₂O, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) in the 12 850-13 380 cm⁻¹ spectral region which is the higher energy region reported so far for this water isotopologue. Very high deuterium enrichment was necessary to minimize the HDO absorption lines overlapping the D₂O spectrum. The achieved sensitivity (noise equivalent absorption α_min ∼ 10⁻⁹ cm⁻¹) allowed detecting transitions with line strengths on the order of 5 × 10⁻²⁸ cm/molecule. The spectrum analysis, based on recent variational calculations has provided a set of 422 new rovibrational energy levels belonging to 11 vibrational states, including rotational sublevels for four new vibrational states and one level of the (0 9 1) highly excited bending state. The very weak (1 0 4)-(0 0 0) band at 13 263.902 cm⁻¹, which is the highest D₂¹⁶O band currently observed, could be assigned despite the fact that the HDO absorption in the region is stronger by three orders of magnitude. The list of 996 D₂¹⁶O transitions is provided as Supplementary Material.     

High sensitivity absorption spectroscopy of HDO by ICLAS-VeCSEL between 9100 and 9640 cm

The absorption spectrum of monodeuterated water has been recorded between 9100 and 9640 cm⁻¹ using intracavity laser absorption spectroscopy (ICLAS) based on a vertical external cavity system emitting laser (VeCSEL). Overall 1706 lines were attributed to the HDO species. The spectrum assignment was performed on the basis of the ab initio calculations by Schwenke and Partridge. A set of 746 energy levels was derived from transitions assigned to 13 upper vibrational states, 300 of them being reported for the first time. Resonance interactions leading to an important strengthening and observations of the very weak 7ν₂ and ν₁ + 5ν₂ bands are discussed. A detailed line list has been generated.     

ICLAS of HDO between 13,020 and 14,115 cm

The high resolution absorption spectrum of monodeuterated water, HDO, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) in the 13,020–14,115 cm⁻¹ region dominated by the 4ν₃ band. The achieved noise equivalent absorption (α_min10⁻⁹ cm⁻¹) allowed detecting transitions with line strengths as small as 2×10⁻²⁷ cm/molecule which is about 10 times lower than the smallest line intensities previously detected in the region.The rovibrational assignment of the spectrum was based on the results of the variational calculations of Schwenke and Partridge (SP) as well as recent calculations using a new potential energy surface performed by Voronin, Tolchenov and Tennyson (VTT). 2157 transitions involving 21 upper vibrational states were assigned to HD¹⁶O while only four bands were previously reported in the region. A set of 157 new energy levels could be derived. It includes rotational levels of several highly excited bending states, in particular the (0 11 0) pure bending state. For some states like the (1 0 3) and (0 2 3) Fermi dyad, effective Hamiltonian modelling was needed to establish the vibrational assignments of some rotational levels. VTT calculations were found to significantly improve the SP results, the rms deviation of the calculated and observed energies being decreased from 0.23 to 0.06 cm⁻¹.Finally, 79 transitions of the 4ν₃ band of the HD¹⁸O isotopologue were assigned, leading to the derivation of 48 levels, which are the most excited energy levels reported so far for this isotopologue.     

Combined analysis of the high sensitivity Fourier transform and ICLAS-VeCSEL absorption spectra of D2O between 8800 and 9520 cm

The absorption spectrum of dideuterated water, D₂O, has been recorded between 8800 and 9520 cm⁻¹ by intracavity laser absorption spectroscopy (ICLAS) based on a vertical external cavity system emitting laser (VeCSEL) and by high sensitivity Fourier Transform spectroscopy. The combined analysis of the spectra has allowed attributing 1223 transitions to the D₂O species. The spectrum assignment was performed on the basis of the recent results of variational calculations based on an optimized potential energy surface of D₂O. A set of 687 energy levels was derived from transitions assigned to eight upper vibrational states, 577 of them being reported for the first time. A detailed line list has been generated. The line intensities were retrieved mainly from the FTS spectrum and the absolute integrated intensities of the 2v₁ + v₂ + v₃ and the v₂ + 3v₃ bands dominating the spectrum have been determined.     

Intracavity laser absorption spectroscopy of HDO between 12 145 and 13 160 cm

The high resolution absorption spectrum of monodeuterated water, HDO, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) in the 12 145-13 160 cm⁻¹ region. The achieved sensitivity (noise equivalent absorption on the order of α_min ∼ 10⁻⁹ cm⁻¹) allowed detecting transitions with line strengths as weak as 10⁻²⁷ cm/molecule which is about 50 times lower than the weakest line intensities previously detected in the considered region.The rovibrational assignment of the 1179 lines attributed to the HDO isotopologue was based on the results of the variational calculations of Schwenke and Partridge as well as the recent calculations based on a new HDO potential energy surface refined from the fitting to the available experimental data. The overall agreement between these new calculations and the observed spectrum is very good, the rms deviation of the differences between the calculated and observed energy values being 0.05 cm⁻¹. A set of 304 new experimental HDO energy levels was obtained. In particular, band origins for the (1 2 2), (2 0 2), and (3 1 1) vibrational states, at 12 568.190, 12 644.652, and 12 919.938 cm⁻¹, respectively, and their rotational sublevels are derived for the first time. A detailed HDO database of 1337 transitions was constructed and is provided as Supplementary Material.     

Rovibrational spectrum of the Ne-N2O van der Waals complex in the 1285 cm region

The rovibrational spectrum of the Ne-N₂O van der Waals complex has been recorded in the symmetric stretching mode region of the N₂O monomer (∼1285 cm⁻¹) using a tunable diode laser spectrometer in conjunction with an astigmatic multi-pass cell and a pulsed supersonic slit jet. The spectra of both ²⁰Ne-N₂O and ²²Ne-N₂O isotopomers are assigned and analyzed using a Watson S-reduced asymmetric-rotor Hamiltonian. The rotational and centrifugal constants for the excited vibrational state are accurately determined. The band-origin of the spectrum is determined to be ν₀ = 1285.12251(18) cm⁻¹ for ²⁰Ne-N₂O and 1285.12363(27) cm⁻¹ for ²²Ne-N₂O, which shows a blue-shift of 0.21921 cm⁻¹ for ²⁰Ne-N₂O and 0.22033 cm⁻¹ for ²²Ne-N₂O from that of the N₂O monomer, respectively.     

Line positions and energy levels of the O substitutions from the HDO/D2O spectra between 5600 and 8800 cm

Absorption spectra of HDO/D₂O mixtures recorded in the 5600-8800 cm⁻¹ region with a total pressure of water from 13 up to 18 hPa and an absorption path length of 600 m have been analyzed in order to obtain new spectroscopic data for HD¹⁸O and D₂¹⁸O. In spite of the low natural ¹⁸O concentration (about 2×10⁻³ with respect to the ¹⁶O one), about 1100 transitions belonging to HD¹⁸O and more than 280 transitions belonging to D₂¹⁸O have been assigned. Most of the D₂¹⁸O transitions belong to the ν₁+ν₂+ν₃ and 2ν₁+ν₃ bands. Sets of energy levels for seven vibrational states of D₂¹⁸O and four states of HD¹⁸O are reported for the first time. The comparison of the experimental data with the calculated values based on Partridge-Schwenke global variational calculations is discussed.     

Fourier transform absorption spectra of H2O and H2O in the 8000-9400 cm spectral region

Fourier transform spectra of water vapor enriched in ¹⁸O and ¹⁷O were recorded between 8012 and 9336 cm⁻¹ and analyzed for the first time. High accuracy ab initio predictions of line positions and intensities by Partridge and Schwenke [J. Chem. Phys. 106 (1997) 4618-4639; 113 (2000) 6592-6597] were used in the process of spectrum assignment. Transitions involving the (031), (111), (130), (210), and (012) upper vibrational states were identified in the recorded spectra. As a result, 514 and 244 precise ro-vibrational energy levels were derived for the H₂¹⁸O and H₂¹⁷O molecules, respectively. High-order resonance perturbations between levels of the vibrational states involved were evidenced leading to the identification of a number of rotational levels of the (050) and (060) highly excited bending states.     

Analysis of the first triad of interacting states (0 2 0), (1 0 0), and (0 0 1) of D2O from hot emission spectra

The far-infrared and middle-infrared emission spectra of deuterated water vapour were measured at temperatures 1370, 1520, and 1940 K in the ranges 320-860 and 1750-3400 cm⁻¹. The measurements were performed in an alumina cell with an effective length of hot gas of about 50 cm. More than 3550 new measured lines for the D₂¹⁶O molecule corresponding to transitions from highly excited rotational levels of the (0 2 0), (1 0 0), and (0 0 1) vibrational states are reported. These new lines correspond to rotational states with higher values of the rotational quantum numbers compared to previously published determinations: J_max = 29 and K_a(max) = 22 for the (0 2 0) state, J_max = 29 and K_a(max) = 25 for the (1 0 0) state, and J_max = 30 and K_a(max) = 23 for the (0 0 1) state. The extended set of 1987 experimental rotational energy levels for the (0 2 0), (1 0 0), and (0 0 1) vibration states including all previously available data has been determined. For the data reduction we used the generating function model. The root mean square (RMS) deviation between observed and calculated values is 0.004 cm⁻¹ for 1952 rovibrational levels of all three vibration states. A comparison of the observed energy levels with the best available values from the literature and with the global predictions from molecular electronic potential energy surfaces of water isotopic species [H. Partridge, D.W. Schwenke, J. Chem. Phys. 106 (1997) 4618] is discussed. The latter confirms a good consistency of mass-dependent DBOC corrections in the PS potential function with new experimental rovibrational data.     

Intracavity laser absorption spectroscopy of HDO between 11 645 and 12 330 cm

The weak absorption spectrum of monodeuterated water, HDO, has been recorded by intracavity laser absorption spectroscopy (ICLAS) between 11 645 and 12 330 cm⁻¹. This spectrum is dominated by the ν₂ + 3ν₃ band of HDO at 11969.76 cm⁻¹. A total of 497 energy levels belonging to 12 vibrational states were determined while only 140 levels were previously reported from a recent investigation by Fourier transform spectroscopy in the same spectral region. The rovibrational identification process of the 1378 lines assigned to the HDO isotopologue was mostly based on the results of the accurate variational calculations of Schwenke and Partridge. The overall agreement between these calculations and the observed spectrum is very good. However, strong discrepancies in the calculated line intensities were evidenced in a few cases corresponding to an intensity transfer to a dark state through local resonance interaction.     

Water vapor absorption line intensities in the 1900-6600 cm region

Water vapor infrared spectra have been measured using the Bruker IFS 120 HR Fourier transform spectrometer at the Physikalisch-Chemisches Institut of the Justus-Liebig-Universität Giessen. Spectra were recorded at pressure-broadening-limited resolution and at room temperature in the range of 1900-6600 cm⁻¹. The use of fully evacuated transfer optics and a White-type multireflection cell made it possible to obtain pressure×pathlength products up to 31.27 mbar×288.5 m. These spectra have previously been used to determine experimental values of rovibrational line positions and upper energy levels of the 2ν₂, ν₁, and ν₃ bands [Mikhailenko SN, Tyuterev VlG, Keppler KA, Winnewisser BP, Winnewisser M, Mellau G, et. al. The 2ν₂ band of water: analysis of new FTS measurements and high-K_a transitions and energy levels. J Mol Spectrosc 1997;184: 330-49] and of the 3ν₂, ν₁+ν₂, and ν₂+ν₃ bands [Mikhailenko SN, Tyuterev VlG, Starikov VI, Albert KK, Winnewisser BP, Winnewisser M, et al. Water spectra in the region 4200-6250 cm⁻¹, extended analysis of ν₁+ν₂, ν₂+ν₃, and 3ν₂ bands and confirmation of highly excited states from flame spectra and from atmospheric long-path observations. J. Mol. Spectrosc. 2002; 213: 91-121].This work presents the intensities of 3769 lines for the weak and medium transitions in the spectral range indicated. These data provide an independent source of experimental information which is complementary to intensity data available in the literature and can thus help to evaluate experimental errors and the reliability of these spectral line parameters.     

Infrared diode laser spectroscopy of the He-N2O van der Waals complex in the 1285 cm region

The rovibrational spectrum of the He-N₂O van der Waals complex has been recorded in the N₂O-monomer ν₁ region (∼1285 cm⁻¹) using an infrared tunable diode laser spectrometer in conjunction with a free supersonic jet expansion and an astigmatic multi-pass absorption cell. Twenty-two lines are assigned to the ν₂-band of He-N₂O. Rotational constants for the ν₂-excited state are derived. The band-origin of the spectrum is determined to be ν₀ = 1285.0666(7) cm⁻¹ and shows a blue-shift of 0.1633(8) cm⁻¹ compared with the N₂O ν₁-band origin.     

Highly sensitive absorption spectroscopy of carbon dioxide by ICLAS-VeCSEL between 8800 and 9530 cm

The absorption spectrum of carbon dioxide has been studied between 8800 and 9530 cm⁻¹ by intracavity laser absorption spectroscopy based on a vertical external cavity surface emitting lasers (VeCSEL). Previous laboratory spectra at high resolution were nearly absent in the considered spectral region. Experiments were carried with natural carbon dioxide and with ¹³C enriched carbon dioxide leading to the determination of the rovibrational parameters of a total of 15 very weak vibrational transitions, including two bands of the ¹⁶O¹³C¹⁸O isotopologue. The observed transitions are assigned to components of the 2ν₁ + 3ν₃ triad and of the much weaker 5ν₁ + ν₃ hexad. Our measured line positions are found in excellent agreement with the predictions of the effective Hamiltonians developed for ¹²C¹⁶O₂ and ¹³C¹⁶O₂ but significant deviations were evidenced for the ¹⁶O¹³C¹⁸O minor isotopologue. The relative band intensities within each polyad are also discussed on the basis of the effective Hamiltonian model.     

High sensitivity CW-cavity ring down spectroscopy of water in the region of the 1.5 μm atmospheric window

The absorption spectrum of natural water vapour around 1.5 μm has been recorded with a typical sensitivity of 5 × 10⁻¹⁰ cm⁻¹ by using a CW-cavity ring down spectroscopy set up based on fibred DFB lasers. A series of 31 DFB lasers has allowed a full coverage of the 6130.8-6748.5 cm⁻¹ (1.63-1.48 μm) region corresponding to the H transparency band of the atmosphere. The line parameters (wavenumber and intensity) of a total of 5190 lines, including 4247 lines of water vapor, were derived by a one by one fit of the lines to a Voigt profile. Different isotopologues of water (H₂¹⁶O, H₂¹⁸O, H₂¹⁷O, and HD¹⁶O) present in natural abundance in the sample contribute to the spectrum. For the main isotopologue, H₂¹⁶O, 2130 lines were measured with line intensities as weak as 10⁻²⁹ cm/molecule while only 926 lines (including a proportion of 30% inaccurate calculated lines) with a minimum intensity of 3 × 10⁻²⁷ cm/molecule are provided by the HITRAN and GEISA databases. Our comparison in the whole 5750-7965 cm⁻¹ region, has also evidenced that an error in the process of conversion of the intensity units from cm⁻²/atm to cm⁻¹/(molecule × cm⁻²) at 296 K, has led to H₂¹⁶O line intensities values listed in the HITRAN-2000 database, systematically 8 % below the original FTS values. The rovibrational assignment was performed on the basis of the ab initio calculations by Schwenke and Partridge with a subsequent refinement and validation using the Ritz combination principle together with all previously measured water transitions relevant to this study. This procedure allowed determining 172, 139, 71, and 115 new energy levels for the H₂¹⁶O, H₂¹⁸O, H₂¹⁷O, and HD¹⁶O isotopologues, respectively. The results are compared with the available databases and discussed in regard of previous investigations by Fourier transform spectroscopy. The spectrum analysis has showed that most of the transitions which cannot be assigned to water are very weak and are due to impurities such as carbon dioxide and ammonia, leaving only about 3% of the observed transitions unassigned. The interest of a detailed knowledge of water absorption for trace detectors developed in the 1.5 μm range is underlined: for instance HDO contributes significantly to the considered spectrum while no HDO line parameters are provided by the HITRAN database.