High resolution optogalvanic spectrum of N2-rotational structure of (11, 7) band in the first positive system

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1 PRAMANA joumal of physics Printed in ndia Voi. 42, No. 3, March 1994 pp High resolution optogalvanic spectrum of N2-rotational structure of (11, 7) band in the first positive system P R SAS KUMAR, S S HARLAL, V P N NAMPOOR and C P G VALLABHAN Laser Division, Department of Physics, Cochin University of Science and Technology, Cochin , ndia MS received l0 August 1993; revised 20 November 1993 Abstract. High resolution optogalvanic spectrum of the (11, 7) band in the first positive system of nitrogen molecule has been recorded from to cm- t. Assignment of 432 rotational lines belonging to the 27 branches of this band has been carried out. Keywords. Optogalvanic spectroscopy; nitrogen molecule PACS Nos 82.80; ntroduction Optogalvanic spectroscopy (OGS) using tunable lasers has proven to be a highly useful and sensitive method for detecting transitions of ecited states of atoms and molecules [1, 2]. n OGS, when the incident laser radiation is resonant with a transition of some species present in the gaseous discharge, a change in impedance of the plasma can occur. This change in impedance of the discharge is monitored as a function of the irradiating laser wavelength. Such laser-induced impedance changes manifest as current changes and can be detected as a signal voltage across a suitable ballast resistance connected in series with the discharge cell. First positive system (Bal"o-A3~ +) of nitrogen molecule is the most prominent band system and has been the subject of many detailed investigations [3, 4]. OGS has been successfully applied to investigate transitions of N2 with low as well as high resolution techniques. OG spectrum of the first positive system of nitrogen molecule in the wavelength region of nm under low resolution has been studied by many researchers [2, 5]. Certain results of the investigations on rotational structure of various band systems of N2 by using this technique have also been reported [6-8]. This paper reports the recording of well resolved rotational lines in the cm-t region of the (11, 7) band of the B31-g-A 3y-+ system using high resolution optogalvanic spectroscopy. 2. Eperimental The eperimental set up employed to record Doppler-limited high resolution OG spectrum is reported earlier [6]. n brief, a positive column N2 discharge was maintained at a current of 2-8( + 0.1)mA and a pressure of 0"8 mbar. Modulated beam from a frequency stabilized single mode ring dye laser (Spectra Physic 380D), at about 225(+ 15)mW power, was aially passed through the cell without allowing to 231

2 P=(1 ~) P R Sasi Kumar et ai P==(14) Pn(12) P,,O 1) P1,--,d" 00 ll0 v- N v- P1 t-- v v v w ae ~ =o.o cm -1 Figure crn -v ern -v m "~ High resolution OG spectrum of some main and satellite lines. fall on the electrodes. The laser was scanned in steps of 30 GHz, and the OG signal generated was detected using a lock-in amplifier which is interfaced with a computer for data acquisition. Figure 1 shows such a spectrum obtained in a single scan. Effect of variation in intensity of the dye laser on scanning was eliminated by normalizing the OG signal intensity with the photo diode output of the dye laser. The wavelength was measured using a commercial wave meter and the frequency of the measured lines is accurate to cm Analysis of the OG spectrum The digitized 0(3 spectrum, which is obtained by joining a number of 30 GHz spectra, and the Fortrat parabola of all branches are shown in figure 2. The first positive system of nitrogen molecule, which corresponds to the transition B3F.-A3y~, consists of three subsystems namely 31"lo(Ft)-A32:, +, 3(F2)--A3Y.+, and J2(F3)- A 3 y+. These transitions are composed of 27 branches with three main and si satellite branches in each of the subsystems. Details of the energy level diagram and the rotational analysis are discussed in literature ['9"1. About 648 rotational lines have been observed and 432 of these have been identified as members of the (11, 7) band belonging to all of the 27 branches of the B3Ne-A 3y.+ system. These are listed in table 1. A few transitions belonging to the (12, 8) band are also observed in this region. The correctness of the J number assignment of rotational line is tested by a number of standard combination relations [9"1. n conclusion, optogalvanic spectrum of (11, 7) band of the B3e-A 3y.+ system of N 2 is recorded and J values of rotational lines were assigned in the spectral region from to 17376cm Pramana - J. Phys., Vol. 42, No. 3, March 1994

3 Optogalvanic spectrum - E ~X X ~ X X ~'~X X X X ~X X X. X X X X ~ 7 --~,...=*, * ~ X ~ "^- ~ X X ~X X ' ~# X X XXX X ~ X X.~1 ] X X XX~ X X X XX X ~ X X *-'~" ~ z i ~ : ~ X ~ X XX - ~X ~ X ~X~ X X X X X X X X X~X ~X X N XXXXX X Ll X Xlllm **:1:11,,..** X *lllll***,, ~ X XX X XX ~ XX X ~ ~X!i o ~".~ - 0 U') r"),_ 0, 0 ~D Pramana - J. Phys., VoL 42, No. 3, March

4 P R Sasi Kumar et al Table 1. dentification of the OG resonances with the transitions of the (11, 7) band of N2: B3g-AaT, + system. rotational J Pal Qtl Rll J P12 Q12 R , " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " ~ " " " " " " " " " ff " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " "03 J Pls Q13 Rla J P21 Q21 S2t " " " " " " " " " " " " " " " "90 Continued 234 Pramana - J. Phys,, Vol. 42, No. 3, March 1994

5 Optooalvanic spectrum Table 1. Continued J P13 Qt3 R3 J P21 Q2t R O " " " " " " " " " " " " @ " " " " " " " " " ~ " " " " " " " " " " " " " " " ~ " " " " " " " " " " " " " " " " " " " " > J P22 Q22 R " " " " " " " " " " " " " " " " " " " " " " " " " " "58 J P23 Q23 R " " " " " " " " " " " " " " Continued Pramna - J. Phys., VoL 42, No. 3, March

6 P R Sasi Kumar et al Table 1. Continued J P22 Q22 R22 J P23 Q23 R " " " " " " " " " " " " ~ " " " " " " " " " " " " " " " " " " J P31 Q31 R31 J P32 Q32 R " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " Pranmua - J. Phys., VoL 42, No. 3, March 1994

7 Optogalvanic spectrum Table 1. Continued J P33 Q33 R ~ Acknowledgement Authors are thankful to the Department of Science and Technology (ndia), for financial assistance. One of the authors (PRS), is grateful to University Grants Commission for research fellowship. References [1] R B Green, R A Keller, G G Luther, P K Schenck and J C Travis, Appl. Phys. Lett. 29, 727 (1976) [2] D Feldman, Opt. Commun. 29, 67 (1979) 3] A Lofthus and P H Krupenie, J. Phys. Chem. Ref. Data 6, 113 (1977) 1'4"1 F Rou, F Michaud and J Verges, J. Mol. Spectrosc. 97, 253 (1983) 1'5"1 G Ullas and S B Rai, Pramana - J. Phys. 36, 647 (1991) 1'6] P R Sasi Kumar, B Ambadi, V P N Nampoori and C P G Vallabhan Pramana - J. Phys. 40, 113 (1993) [7"1 T Suzuki and M Kakimoto, J. Mol. Spectrosc. 93, 423 (1982) 1'8"1 K Miyazaki, H Scheingraber and C R Vidal, Phys. Rev. Lett. 50, 1046 (1983) 1'9"1 G Herzberg, Spectra of diatomic molecules (D Van Nostrand, New York, 1961) Pramana -J. Phys., Vok 42, No. 3, March