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Ye Jia, Wang Hailing, Deng Lunhua. Absorption spectra and isotope shifts of the (2, 0), (3, 1), and (8, 5) bands of the A2Πu–X2Σg+ system of N2+15 in near infrared. Chinese Physics B, 2017, 26(10): 103102  
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Absorption spectra and isotope shifts of the (2, 0), (3, 1), and (8, 5) bands of the A2Πu–X2Σg+ system of N2+15 in near infrared
Ye Jia, Wang Hailing†, , Deng Lunhua‡,
State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China

 

† Corresponding author. E-mail: hlwang@phy.ecnu.edu.cn lhdeng@phy.ecnu.edu.cn

Abstract

The high-resolution absorption spectra of the (2, 0), (3, 1), and (8, 5) bands of the – system of have been recorded by using velocity modulation spectroscopy technique in the near infrared region. The rotational constants of the and states of were derived from the spectroscopic data. The isotope shifts of these bands of the – system of and were also analyzed and discussed.

PACS: 31.30.Gs;33.15.Mt;33.20.-t;33.20.Wr
Keyword:molecular constants;absorption spectrum;isotope shift;N2+15" target=_blank>N2+15
1. Introduction

Molecular nitrogen is the most abundant diatomic molecule in the atmosphere of the Earth and plays a critical role in the evolution of life on the Earth. The spectral investigations of N2 and its ion play a significant role in atmospheric and astrophysical studies. N2 and were observed in the auroral emission and comet tails. The abundance ratios of 15N/14N can be estimated for lunar samples, solar wind, and meteoritic samples.[1]

In the early 1950s, Meinel[2,3] first observed the – system of in the aurora glow. In the following decades, the spectra of the A–X system of have been thoroughly investigated in the laboratory by using several spectroscopic techniques, such as emission grating spectroscopy,[4] laser induced fluorescence,[5] velocity modulation spectroscopy,[6] or Fourier transform emission spectroscopy.[7] However, the spectroscopic investigations of are scarce and fragmentary.

In 1988, Reddy et al.[8] studied the – system of and obtained the rotational constants of the vibration levels v = 0−1 of the X state. Boudjarane et al.[9] determined the hyperfine parameters of the ground and excited states of the B–X transition of the (1, 2) band of . Later, they[10] gave the molecular parameters for the excited and perturbed states of and by combining their LIF data for the B–X band and the Fourier-transform (FT) emission data for the A–X system. In 1997, they[11] extended the study of the B–X system to the (0, 1) band, identified 80 rotational lines and removed the strong correlation between the X and B states to determine the uncorrelated values of the excited B and perturbed A states by combining the LIF data of the B–X system and FT emission data of the A–X system. Until now, only Bernard et al.[12] investigated the (0, 0), (0, 1), (1, 2), (1, 0), (2, 1), (2, 0), and (3, 1) bands of the A–X system of , and gave the rotational constants and molecular equilibrium parameters for the A and X states and the isotope shifts between and

In the present work, we report the spectra of the (8, 5), (3, 1), and (2, 0) bands of the A–X system of , derive the related molecular constants of these states from the experimental spectroscopic data, and give the isotope shifts of these states between and .

2. Experiment

The experimental setup has been described in detail in our previous work.[13] The cations were generated by discharging the mixture gases of and helium. The absorption signal of molecular ions was observed with two forms of modulation which were the phase modulation of the optical field at the radio frequency 480 MHz and the velocity modulation at the audio frequency of 23 kHz acting on the same absorption medium together. The axial electric field imposed a modulation on the drifting velocity of ions. The frequency modulated laser beam at 480 MHz passed through the discharge absorption cell. The output signal from the detector was demodulated by a double balance mixer at a frequency of 480 MHz and then by a lock-in amplifier at the reference frequency of 23 kHz. The laser wavenumber was recorded by an attached wavemeter with a resolution of 0.001 cm−1 and was calibrated using a published molecular I2 line.[14] The absolute wavenumbers were determined to an accuracy of approximately ±0.006 cm−1.

3. Results and discussion

A portion of the observed spectra and the corresponding assignments of the (2, 0) band of the – system of are shown in Fig. 1. The observed line shape of a single transition is the second derivative of a Gaussian type. In the present work, the spectra of in the range of 12294.0 cm−1–12731.2 cm−1 were recorded. However, there are gaps in the spectra in the range of 12342.8 cm−1–12406.1 cm−1 and 12583 cm−1–12600 cm−1 due to the fluctuation of the output energy of our laser.

Fig. 1. (color online) A portion of the Doppler-limited absorption spectra of , , and lines of the (2, 0) band of – system of .
3.1. Rotational analysis

The ground state, , of belongs to the Hund’s case (b). The levels with a given value of the rotational quantum number N are split into two components, F1 () and F2 (), due to the electron spin-rotation coupling. The parities of the rovibronic functions are given following the definitions of Brown and Carrington.[15] The excited state, , obeys the Hund’s case (a) at low J values. Each is split into two components: and . The energy level higher than that of the due to the negative coupling constant Av. For each , a certain J level will split into two sublevels with inverse parities due to the -doubling.

In the present work, all the 12 branches (, , , , , , , , , , , and ) of each band system were observed and analyzed to obtain the molecular constants for each vibration level. The branch of the (8, 5) band has not be measured in this work. The assignment of the transitions follows the selection rules: ; and , or . The transitions are labelled as: .

The Pgopher program[16] was used to fit the experimental data to get the molecular constants of the (8, 5), (3, 1), and (2, 0) bands of the – system of . In total, 677 rotational transition lines were assigned and fitted with the same weight, and shown in supplement part. The band labels, rotational J numbers of the observed lines, and the residual of the band origin are given in Table 1. The molecular constants of Av, Bv, Dv, pv, qv, ADv, and γv of the and states of are listed in Table 2 and Table 3, respectively. Here, Av is the spin–orbit constant, Bv is the molecular rotational constant, Dv is the centrifugal distortion constant, ADv is the centrifugal distortion correction to Av, pv and qv are -doubling parameters, and γv is the spin-rotation parameter.

Table 1.

Summary of the observed bands in the – system of

.
Table 2.

The molecular constants (in unit cm-1) of the state of . Numbers in parentheses represent one standard deviation for the last digit.

.
Table 3.

The molecular constants (in unit cm−1) of the state of . Numbers in parentheses represent one standard deviation for the last digit.

.

In Table 2, comparing our experimental results with other published ones, our molecular constants, Bv and Dv of the state are in agreement with those of Reddy et al.[8] and Bernard et al.[12] except for γv values. For example, the experimental values of the γ parameter for the v = 0 of the state from Reddy et al.,[8] Bernard et al.[12] and our values are , , and , respectively. We fitted the experimental data of Reddy et al. using Pgopher program, and gave a γ value of which is different with Reddy’s result. Bernard et al. pointed out that Reddy’s estimated value of of the X state is from the analysis of the B–X system, the in the upper state, , is not statistically significant within 2σ limits. The and parameters are strongly correlated in the – transitions and only the – can be determined with confidence,[17] thus Reddy’s result of the γ in the state is not realistic. However, in the – transition, the γ in state does not correlate significantly with any parameter in the upper state, so it can be properly determined in experiment. The values of the spin-rotation parameter, γ of Bernard’s and ours are in agreement with each other, Reddy’s and the “Fit” results are inconsistent with that of Bernard’s and ours.

Table 3 lists the molecular constants Av, Bv, Dv, pv, and qv of the state. Our experimental ADv values of the v = 2 and 3 in the state are different with those of Bernard’s[12] both in magnitude and sign. The ADv value is determined by how the pv and qv values have been associated. In Bernard’s paper, they assumed that the state interaction with the state,[12] and fixed the ADv values as 0.38 for both v = 2 and 3 of the .

3.2. Isotope shifts

The observed isotope shifts of the (2, 0), (3, 1), and (8, 5) bands of the A–X system of were obtained directly from the differences between our observed lines of and the reported experimental lines[18] of of this Meniel system.

The calculated isotope shifts were determined using the following expression:

where ν is the transition frequency, v is the vibrational quantum number, “”and “” refer to the upper and lower states, , , and are the equilibrium molecular constants. The value of the isotope parameter, .[19]

The observed and calculated band origins and isotope shifts of the (2, 0), (3, 1), and (8, 5) bands of the A–X system of and are listed in Table 4. The band origins of and are obtained from our previous[18] and the present experimental results, respectively. The measured isotope shifts (labbled with ) are extracted from the experimental values of the band origins of and directly. The calculated isotope shifts are calculatedusing equation (1) with several experimental equilibrium molecular constants.[7,18,20] The observed values of the isotope shifts are in agreement with those of the calculated.

Table 4.

Band origins of the – system of and , and the isotope shifts of .

.
4. Conclusion

The high-resolution rotational spectra of the (2, 0), (3, 1), and (8, 5) bands of the Meinel system – of have been recorded by using velocity modulation spectroscopy technique in this work. In total, 677 transition lines were assigned, rotational structures of these bands were analyzed, and the molecular constants of the and states were estimated and compared with other reported values. Also, the related isotope shifts of the A–X system of and were determined and discussed.

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