Study of H216O and H218O absorption in the 16,460–17,200 cm−1 range using LED-based Fourier transform spectroscopy

Journal of Quantitative Spectroscopy & Radiative Transfer 217 (2018) 170–177

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Study of H2 16 O and H2 18 O absorption in the 16,460–17,200 cm−1 range using LED-based Fourier transform spectroscopy S.N. Mikhailenko a,b, V.I. Serdyukov a, L.N. Sinitsa a,c,∗ a

V.E. Zuev Institute of Atmospheric Optics SB RAS, 1, Academician Zuev Square, Tomsk 634021, Russia Mathematical Physics Department, Tomsk Polytechnic University, 30, Lenina av., Tomsk 634050, Russia c Department of Physics, Tomsk State University, 36, Lenina av., Tomsk 634050, Russia b

a r t i c l e

i n f o

a b s t r a c t The spectra of the Н2 16 О and Н2 18 О vapor have been recorded between 16,460 and 17,200 cm−1 by a Fourier transform spectrometer with spectral resolution of 0.05 cm−1 using high luminance LED light source and 60 cm multipath cell with a path length of 3480 cm. A high signal-to-noise ratio (S/N ≈ 50 0 0) enable us to register the lines with intensities of 1.54 × 10−24 to 2.0 × 10−27 cm/molecule. More than 1300 lines were recorded in the spectrum of the natural abundance water vapor and over 1800 lines in the spectrum of the vapor enriched by 18 O. 422 rotation-vibration energy levels of the H2 18 O molecule have been assigned to twenty vibrational states. The majority of these energies was attributed to the (241), (321), (340), (420), (401), (500), and (123) states. 43 energy levels were tentatively assigned to the (043), (142), (302), (161), (260), and (062) states. Eleven energy levels were labeled as belonging to the (280), (081), (190), (0 10 0), (1 10 0), (0 11 0), and (0 12 0) states. The recorded spectra were compared with the simulations based on the HITRAN2016 database and variational lists. © 2018 Elsevier Ltd. All rights reserved.

Article history: Received 25 April 2018 Revised 28 May 2018 Accepted 28 May 2018 Available online 29 May 2018 Keywords: Fourier transform spectroscopy Water molecule High luminance LED light source Absorption of H2 18 O

1. Introduction This paper is a continuation of our study of the water vapor absorption in the near infrared and visible range using Fourier transform spectroscopy (FTS) with high-luminance LED emitters for the light sources [1,2]. The main goal of the presented work is the investigation of the H2 18 O absorption spectrum between 16,400 and 17,200 cm−1 . The high-resolution water absorption spectra in the frequency range above 16,0 0 0 cm−1 have been recorded and analyzed only in a few studies. Initially, Camy-Peyret et al. [3] at Kitt Peak Solar observatory recorded the spectra of the main isotopic species in the 16,500–25,250 cm−1 range. The experiment employed a Fourier transform spectrometer [4] with spectral resolution of 0.017–0.04 cm−1 in a 6-m multipath absorption cell (total path length was up to 434 m) at the water vapor pressure of up to 18.4 Torr. The authors recorded about 680 lines between 16,500 and 17,200 cm−1 , approximately 400 of which were assigned to rotation-vibration transitions of H2 16 O [3]. Later, the Brussel-Reims team [5–7] studied the long path absorption spectra of the natural water vapor between 90 0 0 and

∗

Corresponding author. E-mail addresses: [email protected] Serdyukov), [email protected] (L.N. Sinitsa).

(S.N.

Mikhailenko),

https://doi.org/10.1016/j.jqsrt.2018.05.032 0022-4073/© 2018 Elsevier Ltd. All rights reserved.

[email protected]

(V.I.

30,0 0 0 cm−1 . The spectra in Refs [5–7] were recorded using a high resolution FTS Bruker 120 M coupled to the White-type absorption cell with a base path of 50 m. The total absorption path length was 602.32 m. A high-pressure Xenon arc lamp was used as the light source. The spectra were recorded at room temperature (about 18 °C) with spectral resolution of 0.06 cm−1 . In Ref [5], about 1500 lines were obtained and over 900 lines assigned to H2 16 O transitions between 16,500 and 17,200 cm−1 . Tolchenov et al. [8] made the most complete analysis of the absorption lines from the spectra [5–7]. According to Ref. [8], more than 16,800 of about 18,200 lines were assigned to four water isotopologues, H2 16 O, H2 18 O, H2 17 O, and HD16 O, in the 9250–25,300 cm−1 range. Unfortunately, neither the observed list nor the assigned lines were published in Ref. [8]. Naus et al. [9] used cavity ring-down spectroscopy (CRDS) to record the water vapor absorption spectrum between 16,550 and 18,0 0 0 cm−1 and determined over 1800 lines with an accuracy of 0.01 cm−1 . Tanaka et al. [10] reported the results of the H2 18 O spectrum measurement in the region of interest. The absorption spectrum of the H2 18 O water vapor was recorded using CRDS between 16,570 and 17,120 cm−1 . About 600 spectral lines were assigned to the H2 18 O molecule. 375 lines were assigned to the rotation-vibration transitions but they were not published in Ref. [10].

S.N. Mikhailenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 217 (2018) 170–177

Naumenko et al. [11] reported the assignment of 516 absorption lines to the H2 17 O molecule from CRDS spectrum in the 16,570– 17,125 cm−1 region. The assigned lines were published as Supplementary Material to Ref. [11]. The absorption spectrum of the deuterated water HD16 O was a subject of several studies [12–16]. Over 200 lines of the 5ν 3 band between 16,745 and 17,612 cm−1 were reported by Bykov et al. [12]. The spectrum [12] was recorded by an acousto-optical spectrometer based on a tunable dye laser. Bertseva et al. [13] and Campargue et al. [14] extended the results of Ref. [12] using intracavity laser absorption spectroscopy (ICLAS). Overall, more than 500 transitions of the 5ν 3 , 2ν 2 + 4ν 3 and ν 1 + 4ν 3 bands were assigned in the 16,300–17,055 cm−1 region. Only 268 out of 682 lines were assigned to the HD16 O transitions in the FTS spectrum of a H2 O/HDO/D2 O vapor mixture between 16,325 and 17,155 cm−1 by Jenouvrier et al. [15]. New measurements of the long path absorption of HDO were carried out by Bach et al. [16] in the 11,50 0–23,0 0 0 cm−1 region. Voronin et al. [17] assigned over 30 0 0 HD16 O lines from these measurements. The current version of the HITRAN database [18] includes 6532 transitions of four water isotopologues, H2 16 O, H2 18 O, H2 17 O, and HD16 O in the 16,460–17,200 cm−1 region. We compare the recorded spectra of the natural and 18 O-enriched water vapor with the simulations based on different line lists in Section 4 of the present paper.

1.00

171

Transmission, natural water vapor

0.99

0.98

0.97

0.96

0.95 16500 1.00

16600

Transmission,

16700

16800

16900

17000

17100

17200

16900

17000

17100

17200

18

O enriched water vapor

0.99

0.98

0.97

2. Experiment and line list preparation All experiments have been conducted using the CREE XPE AMB (λ = 594 nm) LED as a light source because of its high luminance in the 16,50 0–17,20 0 cm−1 range. The LED we have used in our experiment has the power of 1.5 W and fed by the power unit GPR-30600 which has ensured the voltage instability of 0.1 mV and the current instability of less than 4 mA. The spectral width of the diode radiation was about 700 cm−1 . In these conditions, the LED radiation provided an increase of the signal amplitude at 16,800 cm−1 by over 2.5 times as compared to the halogen lamp radiation. In addition, the noise, when using a LED, turns out to be 3 times less than the noise produced by a halogen lamp. Thus, the presented spectrometer demonstrates an 8–9 time increase in the efficiency of the light source as compared to the system based on a halogen lamp [19]. The maximum signal, which provided the required signal-to-noise ratio, has been observed on the path length L = 34.8 m. Having made 10,400 co-added scans, we have been able to obtain a S/N ratio of 50 0 0 which corresponds to minimum absorption coefficient Kth = 1 × 10−7 cm−1 . We used a natural stabilization of the LED temperature starting the measurements in an hour after voltage was applied to the LED. Measurements have shown that fluctuations in the temperature of the emitter do not lead to fluctuations of the spectral contents radiation during the measurement cycle. The spectra were recorded using a multipass White cell with a base length of 60 cm which was connected to a high resolution Bruker IFS 125 M Fourier-transform spectrometer and filled with the H2 18 O vapor to a pressure of 26.5 mbar. Triangular apodization was used and the spectral resolution of 0.05 cm−1 . The design of the multipass vertical absorption cell (60-cm long and a volume of 22 l) is based on the White’s three-mirror configuration improved by Bernstein and Herzberg [20]. The pressure measurements have been performed by the AIR-20 M pressure transducer with the pressure measurable range 0–100 kPa and accuracy of the order of 0.1%. The multipass cell uses wide-band mirrors with silver-coated protective layers of SiO2 and Al2 O3 to ensure the reflection coefficient R to be 96–98% for the wavenumbers of up to 20,0 0 0 cm−1 . The temperature in the 75-m3 measurement room has been stabilized using the air conditioner Midea MSE-24HR with

0.96

0.95 16500

16600

16700

16800

Wavenumber, cm

-1

Fig. 1. Overview of recorded absorption spectra between 16,400 and 17,200 cm−1 of the natural (upper panel) and the 18 O- enriched (lower panel) water vapor.

error less than 1 K, permitting long-time (up to 9 days) spectrum measurements. The radiation after the multipass cell was input to the Fourier spectrometer through the emission channel. The IFS-125 M Fourier transform spectrometer was not put under vacuum. Low air humidity (relative humidity no more than 20%) allowed us to avoid purging the spectrometer with dry nitrogen. To prepare the H2 18 O vapor, we used liquid water enriched to 80% with H2 18 O produced by IZOTOP company. The remaining 20% of the sample consisted of H2 16 O and an insignificant amount of H2 17 O. The Н2 18 О vapor was filled after a preliminary evacuation of the cell. Following the vapor filling, the vapor pressure was stabilizing took place in the course of 2–3 h because of adsorptiondesorption stabilization processes. The room thermo stabilization permitted us to carry out a long-time registration. A silicon diode was used for a photo detector. The registered spectrum was a result of summing up 10,368 interferograms recorded with a resolution 0.05 cm−1 . Prior to the spectrum registration, we performed all spectrometer alignment procedures aimed at providing high quality absorption spectra with maximum symmetric non-overlapping lines. The experimental conditions during the registration of the spectra are given in Table 1. An overview of the recorded spectra is shown in Fig. 1 for the natural (upper panel) and the 18 O-enriched water vapor (lower panel). The full Y-scale in Fig. 1 corresponds to 5% of absorption. The two examples of the quality of recorded spectra in the regions of more strong absorption of H2 18 O are shown in Fig. 2. As seen from Fig. 2, the absorption of about 0.03% at the line center is detectable in the spectra.

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S.N. Mikhailenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 217 (2018) 170–177 Table 1 Experimental conditions of the spectrum registration.

Range, cm−1 Long path, cm Pressure, mbar Temperature, °К Resolution, cm−1 Aperture, mm Scan number Radiation source Estimation of isotopic abundances

1.000

Spectrum 1

Spectrum 2

16,460–17,200 3480 26.5 297 ± 1 0.05 1.1 10,368 CREE XPE AMB λ = 594 nm Natural

16,460–17,200 3480 26.5 297 ± 1 0.05 1.1 10,368 CREE XPE AMB λ = 594 nm H2 16 O – 35%, H2 18 O – 64%, H2 17 O – 1%

Transmission

0.995

0.990

0.985

0.980 16770 1.000

16772

16774

16926

16928

16776

16778

16780

16932

16934

0.995

0.990

0.985

0.980 16924

16930 Wavenumber, cm

-1

Fig. 2. Two examples of recorded spectra around 16,776 and 16,930 cm−1 . The spectrum of natural water is in blue line, absorption of enriched vapor is in olive line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

16,964.7537, and 16,965.1367 cm−1 ) are characterized by the absorption values of less than 0.05% at the peaks. Most of the lines shown in the figure are associated with the 4ν 1 + ν 3 and 5ν 1 bands of the H2 16 O and H2 18 O molecules. The H2 16 O and H2 18 O lines are marked with ‘161’ and ‘181’, respectively. More than 1300 lines in the natural water vapor spectrum and more than 1800 lines in the 18 O enriched spectrum were found using SpectraPlot program [21]. This program was used both for the line list preparation and for the line shape fitting. The Voigt profile was used for the line shape fitting and the self-broadening coefficient was fixed to the value of 0.6 cm−1 /atm for the majority of lines. To calibrate the spectra, we used the positions of 90 lines of H2 16 O stronger than 3 × 10−25 cm−1 /molecule in the range between 16,660 and 17,015 cm−1 . The frequency calibration was performed using the line positions from Ref. [5]. The calibration factor (CF) was 0.999 999 815. The partial pressure of the main isotopologue (see Table 1) was OBS ) determined by comparing the experimental line intensities (SRV in two different spectra. The mean ratio of H2 16 O line intensities obtained from Spectrum 2 to those of Spectrum 1 was about 0.35. Therefore, the H2 16 O abundance in Spectrum 2 was taken to be 35%. The H2 17 O abundance was estimated by comparing the observed line intensities with the values from variational calculations [22,23]. The mean ratio was found to be about 0.01. As a result, the H2 18 O abundance in Spectrum 2 was taken to be 64%. A full list of the lines of Spectrum 2 (see Table 1) with rotation-vibration assignments is provided as Supplementary Material (Suppl Math 1.txt) to the paper. The line list includes the following parameters for each line: observed position (ν OBS , cm−1 ), observed intensity (SRV , cm/molecule at 297 K, accounting for isotopologue abundance in the spectrum), self-broadening parameter (γ self , cm−1 /atm), literature position (ν Ref , cm−1 , for H2 16 O and H2 17 O), literature intensity (SRef , cm/molecule at 296 K, for a 100% abundance), energy of lower level (Elow , cm−1 ), and rotationvibration assignment. Only the first three parameters are given for the 136 unassigned lines. 3. Spectrum assignment

Another demonstration of the quality of the recorded spectra is a comparison of our spectrum and those recorded by Tanaka et al. [10] between 16,808 and 16,818 cm−1 that is shown in Fig. 3. A sample of the spectrum quality and the procedure identifying the isotopic species separation at 16,960 cm−1 is shown in Fig. 4. The spectrum of water vapor enriched with oxygen-18 is plotted in solid olive line. The spectrum of water vapor with the natural concentration of isotopologues is plotted in dashed red line. By superimposing the two spectra, one can unambiguously separate the H2 16 O and H2 18 O lines. All Y-scale in Fig. 4 corresponds to 1.5% of absorption. As it is seen from the picture, the noise level in the shown region does not exceed 0.01%, which corresponds to the S/N ratio of ∼10,0 0 0. The lines marked with an ’o’ symbol (16,956.0077, 16,961.0840, 16,961.2710, 16,963.7311,

Overall, 1683 lines were assigned to 1815 transitions of three water isotopologues, H2 16 O, H2 18 O and H2 17 O, while 136 remained unidentified. As a rule, they are rather weak lines (SRV < 3.5 × 10−26 cm/molecule), mostly at the noise level, but probably a part of them may not relate to the water vapor absorption lines. No HDO lines were found in the recorded spectra due to weakness of the HDO bands in the studied region. The H2 16 O lines were trivially assigned by comparing the HITRAN2016 line list [18] with the line positions of the recorded spectra. No new lines for H2 16 O were found as compared to the previous studies [3,5–8]. The rotation-vibration assignment for H2 16 O was taken from the HITRAN2016 list. The H2 17 O lines were assigned by the same way using the observed lines of Naumenko

S.N. Mikhailenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 217 (2018) 170–177 -6

2.0

173

-1

Loss-rate from CRD experiment in 10 cm (Ref. [10])

1.8

1.6

1.4

1.2 16808 1.00

16810

16812

16814

16816

16818

16814

16816

16818

Transmission

0.99

0.98 16808

16810

16812

Wavenumber, cm

-1

Fig. 3. Comparison of the recorded spectra by CRDS (Ref. [10], upper panel) and FTS (this work, lower panel) around 16,813 cm−1 .

1.000

Transmission o

0.998

oo 181

161 181

161

o

161

o

o

181

161

181

181

0.996

181

0.994

181

161

0.992

161

161

0.990 161

0.988 161

0.986 16956

16958

16960 -1 Wavenumber, cm

16962

16964

Fig. 4. Isotopic separation of the lines by comparing the two different spectra. Natural abundance spectrum is in red dash line, enriched spectrum is in olive solid line. H2 16 O lines are with ‘161’ and H2 18 O lines are with ‘181’. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

et al. [11], corresponding empirical energies of Tennyson et al. [24], and variational line intensities [25] (so called “SP line list”) based on the results of Schwenke and Partridge [22,23]. In addition to Ref. [11], one line at 16,865.0764 cm−1 was assigned to the 4ν 1 + 2ν 2 4 0 4 – 3 1 3 transition. The H2 18 O lines were assigned using SP line list [25] and previously known empirical energies. For more details about SP line list, see Ref. [2]. The lines, which correspond to known energies, were assigned on the first step. On the next step, the strongest lines in the spectrum were associated with those in the calculated line list. These new assignments were confirmed by the combination differences. This procedure was repeated several times, assigning decreasingly weaker lines. Thus, we assigned about 10 0 0 transitions. The SP line list is not good enough in the studied region in the terms of the line positions. The differences between the observed and calculated line positions (dν = ν OBS – ν SP ) reach 0.75 cm−1 . For about one third of the lines, the dν values are within 0.3 cm−1 . It is sufficient to assign at least the strong and

medium lines (SRV > 5 × 10−26 cm/molecule). For 220 assigned transitions, the dν exceeds 0.5 cm−1 . The obtained data were used to determine the upper energy levels. In the end, we were able to determine 422 energies associated with 20 vibration states. General information about our set of the H2 18 O empirical energies is shown in Table 2. The vibrational state notation (V1 V2 V3 ), maximum values of the rotational numbers J and Ka and number of empirical energy levels are given for each of the 20 states. As seen from the table, the major part of the energies was assigned to the (241), (321), (340), (420), (401), (500), and (123) states. 44 energies were tentatively assigned to the (043), (142), (302), (161), (260), and (062) states. 11 energies were labeled as levels of the (280), (081), (190), (0 10 0), (1 10 0), (0 11 0), and (0 12 0) states. The assignment of the last 13 states should thus be considered as preliminary. A complete list of the energies and their comparison with published values is provided as Supplementary Material (Suppl Math 2.txt) to the present paper.

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S.N. Mikhailenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 217 (2018) 170–177 Table 2 State-by-state statistics of the empirical energy levels of H2 18 O. Vibrational state (500) (401) (420) (321) (340) (241) (123) (260) (043) (142)

Maximum J Ka

Number of levels

Vibrational state

Maximum J Ka

Number of levels

12 7 11 8 94 10 4 11 6 84 84 86 73 10 3

86 87 50 56 46 23 19 12 7 7

(302) (161) (062) (081) (280) (190) (0 10 0) (1 10 0) (0 11 0) (0 12 0)

33 75 11 4 66 90 62 76 90 72 30

4 8 6 2 1 2 3 1 1 1

Transmission 1.00

1.000

(B)

0.99

0.995

(A)

0.98 0.990 0.97 0.985 0.96 0.980 16822

16824

16826

16828

16830

16996

16998

17000

17002

17004

-1

Wavenumber, cm

Fig. 5. Two examples of the comparison of the natural abundance absorption spectrum (blue solid line) and the simu-lation using the HITRAN2016 line list (red dashed line) around 16,826 cm−1 (left panel) and 17,0 0 0 cm−1 (right panel). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4. Comparison with the HITRAN database and other studies 4.1. Main isotopologue, H2 16 O The HITRAN2016 database [18] contains 5136 transitions of H2 16 O in the spectral range 16,460–17,200 cm−1 . About 400 line positions were taken from Ref. [8], which were based on the experimental studies [5,7]. More than 3050 line positions were calculated from empirical energy levels recommended by the IUPAC task group [24]. More than 1300 line positions were given by Iouli Gordon [26]. 350 line positions were taken from Bubukina et al. [27]. 1457 line intensities have the experimental origin from Ref. [8], and 3679 line intensities are from the variational calculations of Barber et al. [28] and Lodi et al. [29]. In general, a simulation of water absorption in the 16,460– 17,200 cm−1 range using the HITRAN2016 line list gives satisfactory agreement with our observation of the natural abundance spectrum (Spectrum 1, see Table 1). The left panel of Fig. 5 shows an example of the observed (blue solid line) and the calculated (red dashed line) spectra between 16,821 and 16,831 cm−1 . The picture demonstrates a good agreement of the two spectra. Nevertheless, we discovered several minor disagreements between the experimental and the calculated spectra. One example of such disagreement is given in the right panel of Fig. 5. The spectrum near 17,0 0 0 cm−1 contains at least two lines with significant differences between the observation and the simulation. The first one (A) is the line of the 5ν 1 9 0 9 – 8 1 8 transition at 16,999.415 cm−1 . As seen from the picture, either the line intensity value is underestimated or the self-broadening parameter is overestimated for this transition in the HITRAN2016. The second case (B) is an overestimation of the line intensity for the line at 17,003.861 cm−1 . This is because of the double assignment of this line on the HITRAN2016 line list. The first (correct) assignment is the doublet transitions

11 0 11 – 10 1 10 and 11 1 11 – 10 0 10 of the 5ν 1 band. The total intensity of two transitions is 5.77 × 10−26 cm/molecule. The second (incorrect) assignment is the 4ν 1 + ν 3 11 1 11 – 10 1 10 transition with the line intensity of 7.058 × 10−26 cm/molecule. 4.2. H2 18 O isotopologue Only 682 transitions of H2 18 O are included in the HITRAN line list [18] between 16,460 and 17,200 cm−1 . Note that we were able to assign more than 950 transitions in the same region. All HITRAN2016 transitions take their intensities from variational list of Lodi & Tennyson [30]. The line positions of 394 transitions were calculated using the empirical energy levels, recommended by the IUPAC task group [31] and 288 come from variational calculations by Bubukina et al. [27]. The comparison of the observed spectrum of the enriched water with the results of the simulation using the HITRAN2016 data shows much worse agreement than in case of the natural water (see Fig. 5). The two examples of the comparisons are given in Fig. 6. In these pictures, the simulation is plotted in dashed red line. Five H2 18 O lost lines (marked by (L)) are near 16,680 cm−1 on the left panel. The right panel shows the lost (L) and the overestimated in intensity (O) lines between 16,936 and 16,943 cm−1 . The presence of the lost lines in the simulated spectrum is not surprising because the used line list [18] contains less than 700 H2 18 O transitions as compared to about 10 0 0 assigned in the recorded spectrum. This demonstrates that the list of the HITRAN2016 [18] is incomplete. On the other hand, an overestimation of some lines indicates that variational calculations [30] need to be improved. As mentioned above, the differences between the observed and the SP calculated line positions [25] reach 0.75 cm−1 . The new calculated H2 18 O line list [32] is much better from the point of view

S.N. Mikhailenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 217 (2018) 170–177

1.000

Transmission

175

1.00

(L)

(L) (L)

(O)

(L)

(L) 0.995

(L)

0.99

(O) (L) 0.990

0.98

(L)

0.985 16679

16680

16681

16682

16683

0.97 16685 16936

16684

(O) 16937

16938

16939

16940

16941

16942

-1

Wavenumber, cm

Fig. 6. Two examples of the comparison of the 18 O-enriched absorption spectrum (olive solid line) and the simu-lation using the HITRAN2016 line list (red dashed line) around 16,682 cm−1 (left panel) and 16,940 cm−1 (right panel). The lost lines in the list [18] are marked by ‘L’, the lines overestimated in intensity are marked by ‘O’. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1.00

Transmission

1.00

(L)

(L)

(L)

0.99

0.99

0.98

0.98

0.97 16678

(L)

(L)

(L)

16680

16682

16684

0.97 16702

16686

16704

16706

16708

16710

-1

Wavenumber, cm

Fig. 7. Two examples of the comparison of the 18 O-enriched absorption spectrum (olive solid line) and the simula-tion using the UCL2016 line list [32] (red dashed line) around 16,683 cm−1 (left panel) and 16,707 cm−1 (right panel). The lost lines in calculated list [32] are marked by ‘L’. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1.00

Transmission

1.00

(L)

(L)

(L)

(L) 0.99

0.99

0.98

0.98

0.97

0.97

0.96

(L) (L)

0.96 16766

16768

16770

16772

16774

16898

-1

16900

16902

16904

16906

Wavenumber, cm

Fig. 8. Two examples of the comparison of the 18 O-enriched absorption spectrum (olive solid line) and the simula-tion using the UCL2016 line list [32] (red dashed line) around 16,770 cm−1 (left panel) and 16,902 cm−1 (right panel). The lost lines in calculated list [32] are marked by ‘L’. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of the line positions. We were able to compare the recorded spectrum against the simulation using this new line list. The comparisons of the observed and the calculated spectra are presented in Figs. 7 and 8. As seen from the pictures, the differences between the observed and the calculated positions do not exceed 0.1 cm−1 for majority of the lines. The comparison shows a good agreement between the observed and the calculated intensities for weak and medium lines (see Fig. 7), though for some strong lines the agree-

ment is worse (see Fig. 8). The lost lines in Figs. 7 and 8 are the H2 16 O lines which are not included in the list of Ref. [32]. The most complete list of the H2 18 O energy levels was published by the IUPAC task group [31]. The list [31] contains 136 entries in the energy range 16,750–18,150 cm−1 . All these energies were obtained from the line positions of Tanaka et al. [10]. The differences between our and the published values of the energy levels (see left panel in Fig. 9) are due to a different quality

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S.N. Mikhailenko et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 217 (2018) 170–177 0.2

dE = E

TW

-E

IUPAC

, cm

-1

-1

Energy, cm 18300

0.1 18000

0.0

17700

280 081

17400 -0.1 17100 0120 0110 0100 1100 190

-0.2 16800

-0.3 16800

17000

17200

17400 17600 -1 Energy, cm

17800

18000

16500 18200

043 142 302 161 260 062

241 321 340 420 401 500 123

Vibration state

Fig. 9. Comparison of empirical vibration-rotation energies of H2 18 O obtained in this study and those reported by the IUPAC-TG [31]. Left panel: The differences between our (ETW ) and literature (EIUPAC ) energy values. Right panel: Overview of the empirical energies of this study (horizontal bars) and those recommended in Ref. [31] (open circles).

of the spectra recorded in this work and those of Ref. [10] (see Fig. 3). All 136 but five differences dE = ETW – EIUPAC are within 0.05 cm−1 . The maximum values of dE are for the (500) 4 3 2 (dE = 0.155 cm−1 ) and (500) 8 5 4 (dE = −0.266 cm−1 ) levels. The energy value of the (500) 4 3 2 level (17,150.971 cm−1 ) has been confirmed by the combination difference of the two line positions in our study. In Ref. [10], only a single transition was assigned for this level. It is not possible to confirm our assignment for the (500) 8 5 4 level because all but one corresponding transitions are weak. Contrary to the assignment of the line at 17,008.877 cm−1 in Ref. [10], we assigned the 5ν 1 8 5 4 – 7 4 3 transition to the line at 17,008.6107 cm−1 . Right panel of Fig. 9 shows a general comparison of the two sets of H2 18 O energies in the energy scale between 16,50 0 and 18,50 0 cm−1 . As mentioned above, the labeling to the (043), (142), (302), (161), (260), (062), (280), (081), (190), (0 10 0), (1 10 0), (0 11 0), and (0 12 0) states should be taken as tentative due to a small number of data for these states. 5. Conclusion The absorption spectra of the natural abundance and the 18 O enriched water vapor between 16,460 and 17,200 cm−1 have been recorded at room temperature by a Fourier transform spectrometer with a spectral resolution of 0.05 cm−1 using a high-luminance LED light source and a 60-cm multipath cell with a path length of 3480 cm. The line parameters of about 1700 water lines were obtained using the line shape fitting with Voigt profile. The observed water lines were assigned to three major isotopologues (H2 16 O, H2 18 O and H2 17 O). High enrichment by 18 O enabled us to assign about 10 0 0 transitions of the H2 18 O molecule. These data significantly extend the measured lines for H2 18 O in the region of interest. Assigned transitions allowed determining 422 energy levels associated with 20 vibrational states. This is a significant increase in the number of previously known empirical energies in the energy range above 16,500 cm−1 . The comparisons of recorded spectra with the simulations using the HITRAN2016 [18] and the variational line lists [32] show the limitation of the existed lists. The HITRAN2016 list is good enough for dealing with the absorption of the natural abundance water vapor in the given region. At the same time, this list needs to be completed with new H2 18 O data for the simulations of 18 O- enriched spectra. Despite the fact that the new variational H2 18 O line lists (for example, Refs. [30,32]) are much better than the SP line list [25] in terms of the line positions, the new lists do not have complete vibration-rotation assignments of all included transitions. This is why we used the SP line list for the line assignments of the spectrum under study.

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