Overtone, 2NH (ν1 + ν3) spectroscopy of 15NH3–Ar

Journal of Molecular Spectroscopy 318 (2015) 107–109

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Note

Overtone, 2NH (m1 + m3) spectroscopy of

15

NH3–Ar

T. Vanfleteren a, T. Földes a, M. Herman a,⇑, G. Di Lonardo b, L. Fusina b a b

Laboratoire de Chimie quantique et Photophysique, CP160/09, Faculté des Sciences, Université Libre de Bruxelles, 50, ave. Roosevelt, B-1050, Belgium Dipartimento di Chimica Industriale ‘‘Toso Montanari”, Università di Bologna, Viale Risorgimento, 4, I-40136 Bologna, Italy

a r t i c l e

i n f o

Article history: Received 6 October 2015 In revised form 30 October 2015 Accepted 30 October 2015 Available online 31 October 2015 Keywords: van der Waals complex Ammonia CRDS Overtone spectroscopy

a b s t r a c t We report on the observation of the P (11; 2NH) R (00; ground state) band in 15NH3–Ar, with origin at 6615.943 cm1, using jet-cooled cw-cavity ring-down spectroscopy. The rotational temperature is estimated to be 7 K. Nineteen rotational lines were assigned. Perturbations were evidenced from anomalous line positions and line widths, but not unraveled. Upper state rotational constants were obtained from the analysis of the nine unperturbed R/P lines. The e-symmetry upper state predissociation lifetimes appear to decrease with J0 , from about 1.2 ns to 250 ps from J0 = 1 to 9. Ó 2015 Elsevier Inc. All rights reserved.

1. Observation of a new band in the overtone spectrum of Ar jet-cooled 15NH3 Jet-cooled spectra of 15NH3 were recorded at ULB-Brussels using the cavity ring-down spectroscopy (cw-CRDS) spectrometer in the FANTASIO+ set-up (for ‘‘Fourier trANsform, Tunable diode and quadrupole mAss spectrometers interfaced to a Supersonic expansIOn”) [1,2]. The spectral range contains the m1 + m3 (2NH) band with origin close to 6597 cm1 [3]. The full range recorded extends from 6567 to 6639 cm1. Only a fraction is considered here, corresponding to that of two DFB laser diodes (20 mW, 1 MHz linewidth laser modules by NEL (NLK1556STG) and Sumitomo (SLT5411-CB-S850)). In addition to monomer vibration–rotation lines, we have observed in this restricted range a band, presented in Fig. 1, whose carrier is identified in the next section to be 15NH3–Ar. There seems to be no other spectral feature of this molecular complex reported in the literature, apart from the rotational lines in the ground state (GS) recorded in the microwave range [4]. In the experiment, the cavity was made of two concave mirrors (Layertec 106767, radius = 1000 mm; reflectivity = 99.9985%), separated by about 540 mm, ensuring some 72 000 passes, i.e. about 750 m effective absorption pathlength below the 1 cm long slit, thus in the jet-cooled molecules. Spectral recording was as described in Ref. [5], yielding a noise level amin = 5  1010 cm1 (considering the full length of the cavity as effective absorption pathlength), with a the absorption coefficient defined as in [1,2]. ⇑ Corresponding author. E-mail address: [email protected] (M. Herman). http://dx.doi.org/10.1016/j.jms.2015.10.011 0022-2852/Ó 2015 Elsevier Inc. All rights reserved.

The absolute wavenumber calibration was obtained using a room-temperature 15NH3 spectrum, not further detailed here, recorded using a Fourier transform spectrometer (FTS). We checked that the procedure results in line position accuracy better than 0.001 cm1 for the stronger lines. The gas mixture was prepared by on-line mixing Ar (Air liquide) with a 1% home premixed sample of 15NH3 (99% stated isotopic purity, Euriso-top) and Ar (Air liquide) resulting in a 0.2% 15NH3/Ar mixture, which was injected at a flow rate of 3000 sccm (where sccm denotes cubic centimeter per minute at standard conditions for temperature and pressure). The reservoir (p0) and residual (p1) pressures were kept constant at 115 kPa and 1 Pa, respectively. The rotational temperature (Trot) was estimated to be 7 K, as determined from the evolution of the line intensity within the R and P branches. 2. Assignment of the new band The newly reported structure is assigned to the P (11) R (00) transition in the 2NH GS vibrational band of 15NH3–Ar. In the conventional Mj (jk) notation just used, j is the internal angular momentum, i.e. of the almost freely rotating NH3 unit. Its projection onto the anisotropy axis connecting NH3 and Ar defines Mj (=0 for R; =±1 for P). The quantum number k is the usual projection of j onto the ammonia monomer principal axis of rotation. J, later used in this note, corresponds to the end-over-end rotation of the molecular complex. The observation of this band was expected since the corresponding one had recently been identified in 14NH3–Ar, using very similar experimental conditions [5,6]. The band identification in the main isotopic species is based on the

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T. Vanfleteren et al. / Journal of Molecular Spectroscopy 318 (2015) 107–109

Fig. 1. P (11; 2 NH) R (00; GS) band in 15NH3–Ar (observed and simulated), as recorded using Ar jet-cooled cw-cavity ring-down spectroscopy. Unassigned lines on the figure are from the monomer. The simulation corresponds to a rotational temperature of 7 K. The signs ⁄ and + point out lines perturbed and overlapped by monomer lines, respectively.

closeness of the molecular complex band with origin at 6628.01 cm1 to that of the corresponding transition in the 14 NH3 monomer, i.e. ðDK DJðJ; KÞ ¼Þ rR(0,0), m1 + m3 reported in the literature at 6624.48 cm1 [7]. A more detailed analysis of a broader spectral range was carried over in Ref. [6], supporting this assignment in 14NH3–Ar. The origin of the band presently reported from 15NH3–Ar shows a very close isotopic redshift (12.1 cm1) as that between the corresponding rR(0, 0), m1 + m3 monomer lines (12.3 cm1 [3]). Also, it is the strongest band of those assigned in 14 NH3–Ar, and actually the only one expected to appear in the somewhat weaker spectrum presently recorded using 15NH3. Finally, and even more convincing, the rotational ground state (GS) constants extracted from the present analysis agree with those of the literature for 15NH3–Ar [4]. 3. Rotational analysis of the P (11)

R (00) transition

The expected linear-type PR band structure of the P (11) R (00) transition shown in Fig. 1 was rotationally analyzed using the PGopher software [8]. The resulting constants are listed in Table 1. GS constants from the literature were constrained during the analysis, after checking that they matched the observed GS combination differences. Assigned lines are listed in Table 2. Overlapped and perturbed lines identified in Table 2 were not included in the fit procedure. The band is embedded in usually stronger monomer spectral features. A large overlap occurs for the Q branch just below 6616 cm1, in particular, allowing only one of the lines to be identified and measured in this branch. The assignment of this Q line to the molecular complex rather than to the monomer is supported by the absence of this line in the FTS room temperature spectrum already mentioned. It was assumed that this Q line is not perturbed and the numbering was selected to give optimal Table 1 Spectroscopic constants from the analysis of the P (11; 2 NH) R (00; GS) band of the 15Ar–NH3 van der Waals complex (values in cm1). Only unperturbed R/P lines were considered in the procedure.

B D m~0 No. lines Standard deviation of the fit a

Table 2 List of assigned transitions of the 15Ar–NH3 van der Waals complex in the P (11; 2 NH) R (00; GS) band (observed, calculated and obs-calc values in cm1).

Ground statea

2 NH

0.09235392 2.74  106 0 – –

0.09106(14) 8.7(41)  106 6615.94360(95) 9 9  104

Values constrained to those of Nelson Jr. et al. Ref. [4].

a b c

Observed

Calculated

Obs-calc

6614.1843 – 6614.6272 6614.7930 6614.9927 6615.1889 6615.3802 – 6615.8480 6616.1266 6616.3050 6616.4817 6616.6542 6616.8220 6617.0213 6617.161 6617.3104 6617.4471 6617.5747

6614.1652 6614.3804 6614.5895 6614.7931 6614.9936 6615.1895 6615.3818 6615.5717 – 6616.1252 6616.3049 6616.4814 6616.6539 6616.8211 6616.9846 6617.1417 6617.2917 6617.4334 6617.5655

0.0191 – 0.0377 0.0001 0.0009 0.0006 0.0016 – – 0.0014 0.0001 0.0003 0.0003 0.0009 0.0367 0.0193 0.0187 0.0137 0.0092

Assignment P(9)b P(8)a P(7)b P(6) P(5) P(4) P(3) P(2)a Q(5)c R(0) R(1) R(2) R(3) R(4) R(5)b R(6)b R(7)b R(8)b R(9)b

Overlapped by monomer line. Perturbed lines. Tentative numbering.

agreement with the band origin retrieved from the fit of the R/P lines. The Q line numbering listed in Table 2 remains uncertain. As demonstrated in Fig. 2, R/P lines reaching upper (e-symmetry [9]) levels with J > 5 appear consistently shifted, indicating upper state perturbations. The very complex pattern of vibrational states in the 2NH excitation range in 14NH3 [7,10], and therefore in 15NH3, is not known well enough to allow for interpreting these perturbations in the 15NH3–Ar molecular complex. Further information on the perturbation mechanism can nevertheless be obtained from the analysis of the line widths. As for other molecular complexes investigated using the FANTASIO+ set-up (e.g. [6,11–15]), lines, here from R/P branches, were fitted to a Voigt profile, imposing the Doppler FWHM corresponding to the estimated value of Trot (7 K). The retrieved Lorentzian FWHM was used to extract the upper level predissociation lifetime using the procedure detailed in [11]. The results are displayed in Fig. 3. A J-dependence is observed, very close to that reported in 14NH3–Ar (and actually also in 14NH3–Kr) for the corresponding band, see Fig. 8 in Ref. [6]. The e-upper state predissociation lifetime decreases with J0 , from about 1.2 ns to 250 ps from J0 = 1 to 9.

T. Vanfleteren et al. / Journal of Molecular Spectroscopy 318 (2015) 107–109

Fig. 2. Observed-calculated values from the fitting of R/P lines in the P (11; 2 NH)

Fig. 3. Predissociation lifetimes of the

R (00; GS) band in

109

15

NH3–Ar, in function of the upper state J values.

NH3Ar (upper graph) in e-symmetry rotational levels of the P(11), 2NH (m1 + m3) sub-state.

15

Obviously uncertainties on these values are very high due to the weakness of the lines, at high-J in particular. Error bars on Fig. 3 are retrieved from the lineshape fitting procedure. This J-dependence probably reveals a Coriolis-type interaction affecting the e-series of upper rotational levels. In conclusion, we have reported on the observation of the P (11; 2 NH) R (00; GS) band in 15NH3–Ar using jet-cooled cw-cavity ringdown spectroscopy. Nineteen rotational lines were assigned. Perturbations were evidenced from anomalous line positions and line widths, but not unraveled. Upper state rotational constants were obtained from the analysis of the nine unperturbed R/P lines. The e-symmetry upper state predissociation lifetimes appear to decrease with J0 , from about 1.2 ns to 250 ps from J0 = 1 to 9. The present work, concerning an isotopic form of the ammonia-argon van der Waals complex, thus complements previous investigations of various natures on other forms, more specifically 14ND3–Ar [16–19]. Acknowledgments Thomas Vanfleteren is PhD student with FRIA-Belgium and T. Földes is postdoc with Innivoris, Region Bruxelles Capitale. This work was sponsored at ULB by the ‘‘Wiener-Anspach” foundation (ACME project) and the Brussels-Capital Region within the program Prospective Research for Brussels (Innoviris). It was supported in Italy by the project PRIN2012 ‘‘Spettroscopia e Tecniche computazionali per la ricerca Astrofisica, atmosferica e Radioastronomia” (STAR). The ‘‘Groupe de Recherche International (GDRI) HiResMIR” (High resolution microwave infrared & Raman spectroscopy for molecules of atmospheric, planetologic and astrophysical interest) is also acknowledged.

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