Gopal, R

Joshi, M M

Uttam, K N

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Indian J. Phys. 73B ( 1), 9 7 - 10 0  ( 1999) I NOTEU P  JB—  an in ternationa l journalT h e r m a l e m is s io n  s p e c tr u m  o f  th e  A g C r  m o le c u leK N Uttam, R Gopal and M M JoshiS a h a ’s S pectroscopy  L aboratory , Physics D epartm ent, A llahabad U niversity. A llahabad-211 002, IndiaRet fixed J3 Max 1998. aaepted 29 July 1998A bstract The em ission . pectrum  o f the diatom ic AgCr m olecule has been investigatedby exciting the m ixture o f  silver and chrom ium  metal in a high tem perature graphite furnace in an aigon atm osphere M oic than 25 single headed bands, degraded to shorter wavelength side, have been pho tog raphed  and assigned  to the v ib ia tional schem e o f a sing le  A - X system  Vibrational analysis has been carried out and data reduction leads to the vibrational constants lor the ground state w "  = 222 5 cm wr"x " = 1 25 cm 1 In light of these results, the electronic configurations giving rise to ground state of the AgCr are discussedK e y w o rd s  Spectra of A gCr m olecule, vibrational analysis, d issociation  energy P A C S  N o. 33 20 KlVapour phase intermetal lie molecular systems formed at high temperature has great importance in high temperature chemistry specially in heterogeneous catalysis. Several workers in the past have identified number of diatomic gaseous species through optical spectroscopy. The diatomic AgCr molecule has been identified by Monjajeb 11J by recording Us electronic emission spectrum in the region AA 3680-3900 A. He established ground state vibrational frequency to." for the AgCr molecule is equal to 221.5 cm"1. However, the analysis is based on the spectrogram taken at low dispersion (12-40 A/mm) using Hilgcr Quart/ Spectrograph. Recently, we have published our results about MnSe [2], CoCu [3], and CoSc [4] molecules obtained from the thermal emission, therefore wc decided to investigate the spectra of AgCr molecule at high resolution (0.1 A) and high dispersion (7.3 A/mm) using 2-m Plane Grating Spectrograph.We have been able to record the emission spectrum of AgCr molecule in thermal plasma from high temperature furnace in the spectral range AA 3650-3900 A. We report here the vibrational analysis of A-X system, on the basis of which it is possible to draw some tentative conclusions about the nature of the ground state of AgCr molecule.A small quantity of silver and chromium metal (BDH) was placed inside the carbon tube of Saha’s high temperature graphite furnace [5 ]. After evacuation of the furnace chamber, argon gas was inserted at a pressure of about 40 cm of mercury to prevent rapid effusion of©1999IACS98 K N Uttam, R Copal and M M Joshimolecular vapours from the open ends of the tube. The temperature was raised to 23(X)°C with the help of 10-kVA transformer and resultant glow was recorded using first order of 2-tn plane grating spectrograph with a grating blazed at A 5600 A and total lines 45500. An exposure time of about 4 min was found sufficient to record workable spectra on ILFORD 400 ASA black and white film. An iron arc spectra served comparison standard. The measurement of band heads were performed on a Abbe Comparator with the least count of 0.0001 cm.The thermal emission spectrum of the AgCr molecule consists of 25 single headed, violet degraded bands in the region AA 3650-3900 A. The observed bands have been classified into a single system A-X. The band head data, relative visual estimates of intensity and their vibrational assignment are listed in Table 1. The analysis has yielded following vibrational constants :V  —  (M) 25932.0 a ) '  = 213.0 (O' x 't '  e =  1.8 cm« "  = 222.5 ( 0 " x "t- e=  1.25 cm ' 1T abic 1. Band head data (in cm ’) o f the AgCr molecule A-X systemS No v i(i(cal) v f< (obs) tv', v " ) IntI 271.14.4 2 7 1 3 2  0 (6 .0 ) 12 2 6 9 4 3  0 2 6 9 4 5 .2 (5 ,0 ) 13 2 6 7 4 8  0 2 6 7 4 5  0 (4 ,0 ) 14 2 6 5 4 9  4 2 6 5 4 7  0 (3 ,0 ) 15 2 6 5 2 8  0 2 6 5 3 0  0 (4 .1 ) 16 2 6 5 0 5  5 2 6 5 0 7  0 (5 ,2 ) 17 2 6 3 4 7  2 2 6 3 4 5  0 (2 .0 ) 28 2 6 3 2 9  4 2 6 3 2 8  0 (3 ,1 ) 19. 2 6 2 9 0  5 2 6 2 9 2 .0 (5 ,3 ) 110 2 6 2 6 9  4 2 6 2 7 0  5 (6 .4 ) 11 1 2 6 2 4 7 .2 2 6 2 4 9 .5 (7 ,5 ) 112 2 6 1 4 1 .4 2 6 1 4 2  0 (1 ,0 ) 213 2 6 1 2 7  2 2 6 1 2 9  2 (2 .1 ) 114 26111 9 261 13.5 (3 ,2 ) 115 2 6 0 9 5 .5 2 6 0 9 7 .0 (4 ,3 ) 116 2 6 0 7 8  0 2 6 0 7 9  0 (5 ,4 ) 117 2 6 0 5 9 .4 26061  0 (6 ,5 ) 118. 2 6 0 3 9  7 26041  2 (7 ,6 ) 119. 2 5 9 3 2 .0 2 5 9 3 2 .0 (0 ,0 ) 320. 25921 4 2 5 9 2 3 .0 < u > 221 2 5 9 0 9 .7 2 5 9 0 8  0 (2 ,2 ) 122 2 5 8 9 6 .9 2 5 8 9 8 .2 (3,3) 123. 2 5 8 8 3 .0 2 5 8 8 4  0 (4 ,4 ) 124. 2 5 7 1 2  0 2 5 7 1 4  0 (0 ,1 ) 225 . 2 5 4 9 4 .5 2 5 4 9 6 .5 (0 ,2 ) 2Thermal emission spectrum o f the AgCr molecule 99Since the thermal emission is a low energy excitation, the electronic states involved in the transition must be low lying states. This system may correspond to the A-X system of the AuCr molecule 11] as it lies on the shorter wavele igth side of the corresponding system observed in the case of AuCr. A comparison (shown in Table 2) of the ground state frequency of different gold and silver diatomic combinations with a common element s h o w s  that the ground state frequency of the gold combination was higher than the ground state frequency for the corresponding combination w ith silver, and here also the same trend exists. In the case of the diatomic AuCr and AgCr, we expect the same ground trend to follow, and actually it has been found to be so, as depicted below :w"(AuAI) a/'(AuGa) ft/'(AuIn) cu"(AuSe) co " ( AuCr)1.15 to 1.12o.*"(AgAI) co"(AgGa) " c o " (Agin) c o " { A g S c ) w "(AgCr)T ab le  2. C ollection  o f gold d iatom ics and corresponding silver d iatom icsG old D iatom ic (cm *1) Silver d iato  nic cu"( t (cm ‘) w'\  ,/oj" ,AuAl 3 33 O AgAl 2 5 4  34 1 31AnCI 382  8 AgCl 343  49 1 1 1AuCu 2 5 0  0 AgCu 2 3 1 .8 J 08AuGa 2 2 6  0 AgGa 184 7 1 22Au In 187 A gin 155 54 1 20AuSe 2 7 0 AgSe 2 3 4 1 15AuTe 2 1 2 .0 AgTe 195 3 1 09AuBi 157 0 AgBi 152 14 1 03A uC r 2 52  1* AgCr 222  5# 1 2S*ref I I] , # present study and the rem .jn m g  belong to rel |6 j: normal state electronic configuration of Ag and Cr atom is given by47 Ag : 12 2 *  ft ^  2 ^ 6j? , 2 s  2 p , 3.v 3 p 3 J 10, 4 s 2 4 p !' 4 d ' (). :5 i‘ --------24C r:ls2,2 j 22 /> \3 s23/>63</5, 4.*'----- 7S,The number of possible molecular electronic states predicted by building-up principle when a 2S]r electronic state of silver is combined with a chromium 7S  ^ground state is two :6X, 8Z The molecules CuCr, AgCr, AuCr, and CrH are isovalcnt. Thus, one can expect their molecular orbital to be comparable. In the case of CrH molecule, Ram and Bernath [7] analyzed a 6Z -  6Z transition rotationally and determined spectroscopic constants for both the 6£  term. Therefore in analogy with the observed 6Z state in the case of CrH, we expect ground states character as 6J for the AgCr as well. With 6Z lower state, the possible transitions are 6Z -  6I  and 6I1 -  61. Since the observed bands are appeared to be single headed, therefore in all probability, A-X system involves 6E -  6Z transition.100 K  N  Utiam, R  G opal and  M  M  Josh iThe dissociation energy DQ of the AgCr can be determined by using Birge-Sponer formula. In Birgc formula/),, is given by// 2 " /'D0 =(oe / 4<oe x f ■Substituting values, we find Df = 9901.3 cm '1 = 1.23 eV.AcknowledgmentsThe authors are thankful to Head, Physics Department, Allahabad University, Allahabad for providing necessary facilities to carry out experiments. One of us (K. N. Uttam) is also grateful to Department of Science and Technology, New Delhi for the financial assistance in form of young scientist scheme.References[1] A Monjajeb E l e c t r o n i c  S p e c t r a  o f  C e r t a i n  T r a n s i e n t  G a s e o u s  D t a t o m u s  a t  H t ^ h  T e m p e r a t u r e  (I) Phil Thesis, Allahabad University, Allahabad. India) (1973)[2] K N Uttam, R Gopal and M M Joshi P r a m a n a  43 73 (1994)[3] K N Uttam and M M Joshi I n d i a n  J . P h x s  B68 278 (1994)[4J K N Uttam I n t e r n a t i o n a l  C o n f e r e n c e  o n  S p e c t r o s c o p y  * P e r s p e c t i v e s  a n d  F r o n t i e r s  ( I N C O N S )  (Jan 3-5. 1996, Bhabha Atomic Research Centre, Trombay, Bombay, India) Abstract Booklet p 85 (1996)[5] M N Saha, N K Sur and K Majumdcr Z P h x s  40 648 (1927)[6] K P Huber and G Hcrzberg M o l e c u l a r  S p e c t r a  a n d  M o l e c u l a r  S t r u c t u r e  I V  C o n s t a n t s  o f  D i a t o r n u  M o l e c u l e s  (New York Van Nostrand Remhold) (1979)[7] R S Ram and P F Bemath J  M o l  S p e c t r o s i  172 91 (1995)

Thermal Emission Spectrum of the AgCr Molecule

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Thermal Emission Spectrum of the AgCr Molecule

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