The E'~+-x'Z+ system of GeS in emission

Transcription

1 The E'~+-x'Z+ system of GeS in emission WALTER J. BALFOUR' AND BELVAI J. SHETTY' Departmerzt of Chemistry, University of Victoria, Victoria, BC V8W 3P6, Cnrzada Received March 1 1, 1993 This pc~per is cleclicoted to Professor Gerald W. King on the occasiorz of his 65th birthday WALTER J. BALFOUK and BELVAI J. SHETTY. Can. J. Chem. 71, 1622 (1993). A new group of bands has been observed in emission in the nm region under conditions that generate known systems of GeS. It has been deduced from the appearance and position of the bands, and from their intensity distribution, that these bands constitute the long-wavelength component of the E'C+-X'C+ system of GeS, which gives rise to strong absorption and emission in the ultraviolet. E-X Franck-Condon factors have been estimated. WALTER J. BALFOUR et BELVAI J. SHETTY. Can. J. Chem. 71, 1622 (1993). Optrant dans des conditions qui genkrent des systkmes connus de GeS, on a observt un nouveau groupe de bandes dans la rtgion d'tmission allant de 500 a 600 nm. On dtduit de la presence et de la position des bandes, ainsi que de la distribution de leur intensite, que ces bandes constituent la composante j. grande longueur d'onde du systkme E 'C-X'~~ du GeS qui donne lieu i une forte absorption et emission dans I'UV. On a evalut les facteurs E-X de Franck-Condon. [Traduit par la rtdaction] Introduction ( nm) bands, and the E-X ultraviolet bands, we have In recent years, considerable spectroscopic data have been a new group of bands between 500 and 600 nm. Our gathered on the E'C+-X'C+ system of the isovalent diatom- conjecture that these new bands form the long-wavelength ics SiO (I, 2), SiS (3, 4), and SiSe and SiTe (5, 6). One of component of the E-X system has been confirmed by a the common features of these studies is the observation that combined vibrational analysis of the ultraviolet and visible bands belonging to the E-X system fall in two distinct spec- bands and associated Franck-Condon calculations. tral regions,-one in the ultraviolet and the other in the visi- Experimental details ble. The bands in the visible region appear only in emission. The spectrum of the germanium monosulfide molecule was These observations are a direct result of the substantial change excited in a microwave discharge (2450 MHz, 100 W) using a 5 cm in geometry that occurs upon excitation from the x'c' state long sealed quartz tube (10 mm diameter) containing milligram to the E'C+ state: the ultraviolet bands constitute one arm of quantities of pure elemental germanium (in slight stoichiometric the Condon parabola, the visible bands the other. excess) and sulfur, along with neon gas at a pressure of 2-4 Torr The corresponding germanium compounds can also be (1 Torr = Pa). To maintain a steady vapour pressure of GeS expected to exhibit similar spectroscopic properties. How- in the discharge column the sealed tube was heated with an elecever, to date the bands belonging to the E-X system of ger- trical wire furnace wound around a quartz tube supporting the sealed manium compounds have been reported only in the ultraviolet tube. Further experimental details and advantages of the method region. Of these, the GeS molecule has been the subject of can be found in a previous publication (5). The E-X bands of GeS interest of many researchers. Two ultraviolet band systems in the ultraviolet ( nm) region were photographed in the first order of a 3.4-m Ebert grating spectrograph with a groves of GeS, ~ln-x'c+ and E'Z+-x'C+, are known and have mm-' grating blazed at 750 nm. The bands in the visible ( been vibrationally analysed (7, 8). The rotational analysis of nm) region were photographed in the first order of a 1.5-m Bairdthese band systems is made difficult because of the overlap- Atomic grating spectrograph. The band-heads were measured ping of bands due to significant natural abundances of the against the iron lines excited in a Fe-Ne hollow cathode lamp. isotopomers "G~~'S, '*G~~'s, 73~e3'~, "G~~*s, and 76~e3'~. More recently Magat et al. (9) have achieved the rotational analysis of a few bands belonging to the A-X system by making use of the accurate ground state rotational constants available from microwave spectral studies (10, 1 I). In Drummond and Barrow's absorption study of the E-X system (8) a ur-0 progression was observed extending to u' = 36. However, the vibrational constants they reported were derived from a data set restricted to u' Linton (12) observed two new transitions, b3nl-x'c+ ( nm) and a3c+-x'c' ( nm), in experiments that were completely free from the previously known AT-x'C+ and E'C+x'C' band systems: a chemiluminescent flame produced by the reaction of Ge with OCS. We have examined the GeS spectrum in emission between 200 and 650 nm and, in addition to the known A-X '~uthor to whom correspondence may be addressed. 'on leave ( ) from: Spectroscopy Division, Bhabha Atomic Research Centre, Bombay , India. Results and discussion In the Dresent studies of GeS in emission we have observed extensive band structure throughout the visible and ultraviolet, including the A'n-xlC+ and E'C+-X'C+ systems, but not the bands found by Linton (12) in chemiluminescence. Our spectra, in the ultraviolet down to 220 nm, include bands reported by Drummond and Barrow (8) up to u = 21 of the E-state. Their observations extend somewhat further towards the convergence limit near 214 nm. Some 30 bands in the nm region have been observed that have not previously been reported. Their occurrence together with known GeS bands, and the complete absence of any identifiable impurity spectra, suggested that the new bands also belong to GeS. They are red-degraded and have a similar appearance to the E-X bands in the ultraviolet. We have considered the probability that these new visible bands constitute part of the E-X system and find that they can indeed be integrated in a satisfactory way. The detailed vibrational analysis of the ultraviolet and the

2 BALFOUR AND SHETTY 1623 TABLE 1. Observed band-heads in the E'Zf-x'C+ system of GeS TABLE 1 (co~zcl~ided) in both the ultraviolet and the visible spectral re,' OIO~S Band hair V W Band hair V,;,, Band hair ',ac Band hair VV,, (71'-~") (nrn) (cn1c') (ul-u") (nrn) (crn') (-I") (nm) (cin') (vf-v") (nm) (cin-') (0) Bands in the ultraviolet (b) Bands in the visible visible bands has revealed that while the former arise from transitional between vibrational levels 0 5 v -i 21 in the E'C+ state and 0 5 v 5 8 in the x'c+ state, the latter involve transitions from 33 5 v 5 40 in the E'C+ state to 60 5 v 5 68 in the x'c' state of GeS. A least-squares fit, combining all of our observed band-heads in the ultraviolet and visible region, yielded the following constants: TL = (1.37), ol = (19), ofxl = 1.377(8), = (26) and o:xz = 1.800(4) (cm-i). Values in parentheses are standard deviations. These constants from the global fit of E-X bands may be compared with those obtained from measurements of the first few vibrational intervals in the ground state from rotational analysis of A-X bands (9) and infrared emission spectra (13), where the v = 1 to v = 0 interval was found to be cm-i. The constants above give cm-i, in excellent agreement. The standard deviation of the fit (4.8 cm-i) is well within the experimental uncertainties in band-head positions. Unlike the bands in the A-X system, the bands in the E-X system do not show sharp, well-defined heads. A major source of the uncertainty in the measured band-heads comes from overlapping, unresolved, isotopic structure. For the same reason no isotopic information useful to the assigning of specific vibrational quantum numbers is evident. Band-heads and their vibrational assignments are listed in Table 1. We have used Franck-Condon calculations (14) to verify the above interpretation of the visible GeS bands. The vibrational constants necessary to compute the Rydberg-Klein- Rees (RKR) potential energy curves were taken from the present study. Equilibrium rotational data (B, and a,) for the ground electronic state are available from microwave studies (10, 1 I), but similar data for the E-state have not been measured. Initially we estimated BS and a: parameters by assuming percentage changes upon excitation to be the same for GeS as measured for the isovalent SiS (15). The estimated I-: was nm. We subsequently adjusted the r: value to nm, while preserving the shape of the potential curve, to match the calculated relative intensity distribution of the a'-0 progression to experiment. The RKR curves used are given in Table 2. The Franck-Condon factors obtained from these curves are shown in Table 3. The predictions of these calculations agree well with the observations in both the ultraviolet and visible regions.

3 CAN. J. CHEM. VOL. 71, 1993 TABLE 2. RKR potential energy curves for the x'z' and E'Z' states of GeS X'Z' State E'Z+ State B, (cm-') r i n r n m v G(u) (cm-') B, (cm-') I

4 BALFOUR AND SHETTY 1625 TABLE 2 (concluded) x'c+ State u G(u) (cm-') Bv (cm-') rnmin (nm) E'Z' State r m m u G(u) (cm-') B, (cm-') rmin (nm) rmdx (nm) (a) Ultraviolet bands TABLE 3. Franck-Condon factors in the EIC+-x'C+ system of GeS u' , (b) Visible bands u" 71)' Acknowledgement 1. N. Elander and A. Lagerqvist. Phys. Scr. 3, 267 (1971). This research was supported by funds from the Univer- 2. R.F. Barrow and T.J. Stone. J. Phys. B: At. Mol. Phys. 8, L13 sity of Victoria and the Natural Sciences and Engineering (1975). Research Council of Canada. 3. E.E. Vago and R.F. Barrow. Proc. Phys. Soc. 58, 538 (1946).

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