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A New Transition in the Spectrum of YCl: Rotational Analysis of theK1Π–X1Σ+UV Band System
JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.
175, 217–219 (1996)
0026
A New Transition in the Spectrum of YCl: Rotational Analysis of the K1P –X1S/ UV Band System Ju Xin and Lennart Klynning Department of Physics, Stockholm University, Box 6730, S-113 85 Stockholm, Sweden Received November 29, 1994; in revised form March 7, 1995
The absorption spectrum of the yttrium monochloride molecule (YCl) produced in a King-type furnace has been recorded at high resolution using a 5-m Fastie spectrograph. A new band system in the UV region (centered at 3291 ˚ ) has been found and rotationally analyzed. The transition has been assigned to K1P –X1S/, in accordance with the A labeling of the YCl electronic states by Langhoff et al. (J. Chem. Phys. 89, 396–407, 1988) in their theoretical work. Molecular constants for the new state are presented. q 1996 Academic Press, Inc. 1. INTRODUCTION
From high-resolution spectroscopic data collected so far on the diatomic molecule YCl, six singlet states have been rotationally characterized. These are the ground state X1S/ and the A1D, B1P, C1S/, D1P, and J1P states. The first spectroscopic investigation of YCl was carried out in 1966 by Janney (1), who observed and rotationally analyzed a band system in the red. This transition was later assigned to C1S/ –X1S/ by Huber and Herzberg (2). In 1980, Fischell et al. (3) observed two new band systems in the blue and the near UV using laser-induced fluorescence techniques (LIF) but no precise rotational analyses were given. These two bands were later assigned to D1P –X1S/ and J1P –X1S/ by Langhoff et al. (4) in their theoretical study of the scandium and yttrium halides. Shortly after Fischell et al., Gopal et al. (5) photographed the thermal emission spectra of YCl ˚ , where the in the regions 4065–4645 and 3625–3855 A 1 1 / 1 1 / D P –X S and J P –X S bands should be found. Four band systems were assigned and vibrationally analyzed. However, their vibrational analysis is not consistent with the LIF work of Fischell et al. Apparently, their assignment of the four band systems is incorrect. Since 1990 we have studied the spectrum of the YCl molecule using high-resolution grating spectroscopy, Fourier transform spectroscopy, and various laser-induced fluorescence techniques. Two new electronic states, the B1P and A1D states, have been characterized from the observations and from the rotational analyses of the B1P –X1S/ and D1P – A1D band systems in the near-infrared region. All the other known transitions, C1S/ –X1S/, D1P –X1S/, and J1P – X1S/, which were not rotationally analyzed (or were incorrectly analyzed) previously, have been rotationally analyzed and molecular constants for all these states have been determined (6–8). Experimental investigations of YCl have also been carried
out in other laboratories. Simard et al. (9) investigated the (0, 0) band of the C1S/ –X1S/ system using molecular beam Stark spectroscopy and obtained the permanent dipole moments for the X1S/ and C1S/ states. The molecular orbital structure of YCl states has also been discussed. Very recently, Hensel and Gerry (10) recorded the pure rotational spectrum of YCl using a cavity pulsed microwave Fourier transform spectrometer and determined the chlorine nuclear hyperfine constants. In this study, we report a newly observed electronic transition in the absorption spectrum of YCl and present a rotational analysis of this band system. 2. EXPERIMENTAL DETAILS
The experimental conditions were much the same as described in Ref. (7). Gaseous YCl was produced in a King furnace by heating a 2:1 mixture of YCl3 and Y metal powder to a temperature of 2300 K. The furnace was filled with argon gas at a pressure of 350 Torr in order to prevent rapid effusion of molecules from the heating zone. After a while, the pressure increased to 450 Torr due to the vaporization of the material. The absorption spectrum of YCl was photographed in the ˚ with a 5-m Fastie spectrograph using region 3000–3400 A Kodak 103a-O plates. A xenon lamp served as the light source for the background continuum. The spectrograph was equipped with an order sorter for predispersion. The spectrum was recorded in the 18th order using the grating near the blaze wavelength. The final, ‘‘effective’’ resolution on the photographic plate was about 0.06 cm01. The exposure times were around 2 min for the recorded absorption bands. Thorium atomic lines from a hollow cathode lamp were used for calibration. Wavelengths for the Th spectrum were taken from Palmer and Engleman (11). The photographic plates were measured with an Abbe
217 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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XIN AND KLYNNING
TABLE 1 Wavelengths (in A˚) of the R Heads in the K1P –X1S/ Band System of Y35Cl
where the constants q and qD describe the L-type doubling relative to the e levels. In the final fit, 1400 band lines were reduced to 14 molecular constants. The rms error in the fit was 0.022 cm01. Molecular constants for the K1P state are presented in Table 2. Wavenumbers for all the band lines have been placed in the Depository of Unpublished Data at the editorial office of this journal and are also available from the authors as an internal report. 4. DISCUSSION
comparator equipped with a Heidenhain scale. The rms error obtained when a third degree polynomial was fitted to the ˚ and the corresponding reference lines was typically 0.0005 A accuracy of the line positions was about 0.005 cm01. 3. OBSERVATION AND ANALYSIS
Although the K1P –X1S/ absorption bands are weaker than the bands of the D1P –X1S/ and J1P –X1S/ systems (7), they still have appreciable intensity. We recorded the D£ Å 0, 1, 2, and 01 sequences but only the (0, 0), (1, 0), (0, 1), and (2, 0) bands were strong enough for branches to be picked out. Each band consists of a strong Q branch and weaker R and P branches, apparently due to a 1P – 1S/ transition. No perturbations were found. The transition was found to originate from the ground X1S/ state and was assigned to K1P –X1S/. All bands are degraded toward the red. Wavelengths of the R heads of these bands are presented in Table 1. Only the Y35Cl isotopomer was treated. The analysis and determination of molecular constants were carried out as described in Ref. (7). The wavenumbers of the band lines were fitted using a nonlinear least-squares method. The line positions were calculated using the expression £ Å T* 0 T 9.
[1]
In 1988, Langhoff et al. (4) carried out an ab initio calculation on scandium and yttrium monohalides; a CASSCF / MRCI (complete-active space self-consistent-field plus multireference configuration interaction) study of the spectroscopic properties and the radiative lifetimes was presented for the singlet as well as the triplet states of ScF and YCl below 28 000 cm01. This theoretical study has shown good agreement with the experimental results for YCl. Unfortunately, the state reported here is slightly beyond the region of their calculations. Nevertheless, the assignment of the transition to 1P –X1S/ is obvious and the labeling of the upper 1P state as K1P is tentative but at the moment the most probable choice. According to Langhoff et al. (4), in their study of the excited states of YCl, the active space contained the 3s, 2p, and 1d molecular orbitals (MOs), in the notation which counts only the valence orbitals, equivalent to 15s, 7p, and 2d in order of energy, respectively. The 3s and 2p MOs are predominantly antibonding while the 1d is essentially nonbonding; see Ref. (9). Almost all the low-lying (below 30 000 cm01) electronic states of YCl can be interpreted using this molecular orbital scheme. The highest electronic state within the limit of their calculations, the J1P state, located at Te Å 27 125 cm01 (7), has the leading configurations of 1s21p43s2p, 1s21p42s3p, . . . , where the 1s and 1p are the bonding orbitals, the 2s is essentially a nonbonding
The molecular constants of the ground state X1S/ were kept fixed at the values of Ref. (7) in the direct-approach calculation. Term values for the e levels of K1P and X1S were described by the formula
TABLE 2 Molecular Constants (in cm01) for the K1P State of Y35Cl
Te(£, J) Å Te / [ve 0 vexe(£ / 1/2)](£ / 1/2) / [Be 0 ae(£ / 1/2)]J(J / 1) 2
[2] 2
0 [De / be(£ / 1/2)]J (J / 1)
and the f levels of K1P by Tf(£, J) Å Te(£, J) / qJ(J / 1) 0 qDJ2(J / 1)2,
[3]
Copyright q 1996 by Academic Press, Inc.
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219
THE K1P –X1S/ TRANSITION OF YCl
TABLE 3 A Comparison between the ab Initio Study and the Experiments—Molecular Constants for the 1P States
Note. All values are given in cm01 except the re values which are given ˚. in A
yttrium 5s/5p hybrid, mainly localized on the yttrium atom, while the 3p is mostly the yttrium 5pp orbital and should be less antibonding than the 2p orbital. Comparing the rotational constants of the J1P and K1P states (Table 3), the Be value of the K1P state is smaller (the bond length re is accordingly longer) than that of the J1P state, which suggests that the electron configuration responsible for the K1P state has more antibonding character. In this MO picture, the 3s2p seems to be the most antibonding configuration. It is highly possible that the K1P state (Te Å 30 401 cm01) has the dominant configuration 3s2p, while in the J1P state, 2s3p should be the leading configuration instead of 3s2p, which Langhoff et al. suggested. As can be seen, both the J1P –X1S/ and K1P –X1S/ transitions contain large proportions of the 5p–5s character of the Y/ atomic transition.
The L-type doubling in the K1P state is surprisingly large compared with other 1P states (B1P, D1P, and J1P), about one order of magnitude larger than the L-type doubling in the B1P and D1P states; see Table 3. The negative value of the L-type doubling constant q for the K1P state shows that the f levels lie below the e levels. These two factors suggest that there is either a close-lying 1S/ state just below the K1P state or a 1S0 state just above the K1P state, which can originate for example from 2s4s (1S/), 2p2 (1S/), or 2p3p (1S/ and 1S0), respectively. REFERENCES 1. G. M. Janney, J. Opt. Soc. Am. 56, 1706–1711 (1966). 2. K. P. Huber and G. Herzberg, ‘‘Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules,’’ pp. 672–673, Van Nostrand–Reinhold, New York, 1979. 3. D. R. Fischell, H. C. Brayman, and T. A. Cool, J. Chem. Phys. 73, 4260–4272 (1980). 4. S. R. Langhoff, C. W. Bauschlicher, Jr., and H. Partridge, J. Chem. Phys. 89, 396–407 (1988). 5. R. Gopal, L. K. Singh, and M. M. Joshi, J. Mol. Spectrosc. 89, 15–18 (1981). 6. J. Xin, G. Edvinsson, and L. Klynning, J. Mol. Spectrosc. 148, 59–64 (1991). 7. J. Xin, G. Edvinsson, and L. Klynning, Phys. Scr. 47, 75–81 (1993). 8. J. Xin, G. Edvinsson, L. Klynning, and P. Royen, J. Mol. Spectrosc. 158, 14–20 (1993). 9. B. Simard, A. M. James, and P. A. Hackett, J. Chem. Phys. 96(4), 2565–2572 (1992). 10. K. D. Hensel and M. C. L. Gerry, J. Mol. Spectrosc. 166, 304–309 (1994). 11. B. A. Palmer and R. Engleman, Jr., ‘‘Atlas of the Thorium Spectrum,’’ Los Alamos, New Mexico (unpublished).
Copyright q 1996 by Academic Press, Inc.
AID
JMS 6922
/
m4652$$$90
01-18-96 00:53:31
mspa
AP: Mol Spec
175, 217–219 (1996)
0026
A New Transition in the Spectrum of YCl: Rotational Analysis of the K1P –X1S/ UV Band System Ju Xin and Lennart Klynning Department of Physics, Stockholm University, Box 6730, S-113 85 Stockholm, Sweden Received November 29, 1994; in revised form March 7, 1995
The absorption spectrum of the yttrium monochloride molecule (YCl) produced in a King-type furnace has been recorded at high resolution using a 5-m Fastie spectrograph. A new band system in the UV region (centered at 3291 ˚ ) has been found and rotationally analyzed. The transition has been assigned to K1P –X1S/, in accordance with the A labeling of the YCl electronic states by Langhoff et al. (J. Chem. Phys. 89, 396–407, 1988) in their theoretical work. Molecular constants for the new state are presented. q 1996 Academic Press, Inc. 1. INTRODUCTION
From high-resolution spectroscopic data collected so far on the diatomic molecule YCl, six singlet states have been rotationally characterized. These are the ground state X1S/ and the A1D, B1P, C1S/, D1P, and J1P states. The first spectroscopic investigation of YCl was carried out in 1966 by Janney (1), who observed and rotationally analyzed a band system in the red. This transition was later assigned to C1S/ –X1S/ by Huber and Herzberg (2). In 1980, Fischell et al. (3) observed two new band systems in the blue and the near UV using laser-induced fluorescence techniques (LIF) but no precise rotational analyses were given. These two bands were later assigned to D1P –X1S/ and J1P –X1S/ by Langhoff et al. (4) in their theoretical study of the scandium and yttrium halides. Shortly after Fischell et al., Gopal et al. (5) photographed the thermal emission spectra of YCl ˚ , where the in the regions 4065–4645 and 3625–3855 A 1 1 / 1 1 / D P –X S and J P –X S bands should be found. Four band systems were assigned and vibrationally analyzed. However, their vibrational analysis is not consistent with the LIF work of Fischell et al. Apparently, their assignment of the four band systems is incorrect. Since 1990 we have studied the spectrum of the YCl molecule using high-resolution grating spectroscopy, Fourier transform spectroscopy, and various laser-induced fluorescence techniques. Two new electronic states, the B1P and A1D states, have been characterized from the observations and from the rotational analyses of the B1P –X1S/ and D1P – A1D band systems in the near-infrared region. All the other known transitions, C1S/ –X1S/, D1P –X1S/, and J1P – X1S/, which were not rotationally analyzed (or were incorrectly analyzed) previously, have been rotationally analyzed and molecular constants for all these states have been determined (6–8). Experimental investigations of YCl have also been carried
out in other laboratories. Simard et al. (9) investigated the (0, 0) band of the C1S/ –X1S/ system using molecular beam Stark spectroscopy and obtained the permanent dipole moments for the X1S/ and C1S/ states. The molecular orbital structure of YCl states has also been discussed. Very recently, Hensel and Gerry (10) recorded the pure rotational spectrum of YCl using a cavity pulsed microwave Fourier transform spectrometer and determined the chlorine nuclear hyperfine constants. In this study, we report a newly observed electronic transition in the absorption spectrum of YCl and present a rotational analysis of this band system. 2. EXPERIMENTAL DETAILS
The experimental conditions were much the same as described in Ref. (7). Gaseous YCl was produced in a King furnace by heating a 2:1 mixture of YCl3 and Y metal powder to a temperature of 2300 K. The furnace was filled with argon gas at a pressure of 350 Torr in order to prevent rapid effusion of molecules from the heating zone. After a while, the pressure increased to 450 Torr due to the vaporization of the material. The absorption spectrum of YCl was photographed in the ˚ with a 5-m Fastie spectrograph using region 3000–3400 A Kodak 103a-O plates. A xenon lamp served as the light source for the background continuum. The spectrograph was equipped with an order sorter for predispersion. The spectrum was recorded in the 18th order using the grating near the blaze wavelength. The final, ‘‘effective’’ resolution on the photographic plate was about 0.06 cm01. The exposure times were around 2 min for the recorded absorption bands. Thorium atomic lines from a hollow cathode lamp were used for calibration. Wavelengths for the Th spectrum were taken from Palmer and Engleman (11). The photographic plates were measured with an Abbe
217 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AID
JMS 6922
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m4652$$$89
01-18-96 00:53:31
mspa
AP: Mol Spec
218
XIN AND KLYNNING
TABLE 1 Wavelengths (in A˚) of the R Heads in the K1P –X1S/ Band System of Y35Cl
where the constants q and qD describe the L-type doubling relative to the e levels. In the final fit, 1400 band lines were reduced to 14 molecular constants. The rms error in the fit was 0.022 cm01. Molecular constants for the K1P state are presented in Table 2. Wavenumbers for all the band lines have been placed in the Depository of Unpublished Data at the editorial office of this journal and are also available from the authors as an internal report. 4. DISCUSSION
comparator equipped with a Heidenhain scale. The rms error obtained when a third degree polynomial was fitted to the ˚ and the corresponding reference lines was typically 0.0005 A accuracy of the line positions was about 0.005 cm01. 3. OBSERVATION AND ANALYSIS
Although the K1P –X1S/ absorption bands are weaker than the bands of the D1P –X1S/ and J1P –X1S/ systems (7), they still have appreciable intensity. We recorded the D£ Å 0, 1, 2, and 01 sequences but only the (0, 0), (1, 0), (0, 1), and (2, 0) bands were strong enough for branches to be picked out. Each band consists of a strong Q branch and weaker R and P branches, apparently due to a 1P – 1S/ transition. No perturbations were found. The transition was found to originate from the ground X1S/ state and was assigned to K1P –X1S/. All bands are degraded toward the red. Wavelengths of the R heads of these bands are presented in Table 1. Only the Y35Cl isotopomer was treated. The analysis and determination of molecular constants were carried out as described in Ref. (7). The wavenumbers of the band lines were fitted using a nonlinear least-squares method. The line positions were calculated using the expression £ Å T* 0 T 9.
[1]
In 1988, Langhoff et al. (4) carried out an ab initio calculation on scandium and yttrium monohalides; a CASSCF / MRCI (complete-active space self-consistent-field plus multireference configuration interaction) study of the spectroscopic properties and the radiative lifetimes was presented for the singlet as well as the triplet states of ScF and YCl below 28 000 cm01. This theoretical study has shown good agreement with the experimental results for YCl. Unfortunately, the state reported here is slightly beyond the region of their calculations. Nevertheless, the assignment of the transition to 1P –X1S/ is obvious and the labeling of the upper 1P state as K1P is tentative but at the moment the most probable choice. According to Langhoff et al. (4), in their study of the excited states of YCl, the active space contained the 3s, 2p, and 1d molecular orbitals (MOs), in the notation which counts only the valence orbitals, equivalent to 15s, 7p, and 2d in order of energy, respectively. The 3s and 2p MOs are predominantly antibonding while the 1d is essentially nonbonding; see Ref. (9). Almost all the low-lying (below 30 000 cm01) electronic states of YCl can be interpreted using this molecular orbital scheme. The highest electronic state within the limit of their calculations, the J1P state, located at Te Å 27 125 cm01 (7), has the leading configurations of 1s21p43s2p, 1s21p42s3p, . . . , where the 1s and 1p are the bonding orbitals, the 2s is essentially a nonbonding
The molecular constants of the ground state X1S/ were kept fixed at the values of Ref. (7) in the direct-approach calculation. Term values for the e levels of K1P and X1S were described by the formula
TABLE 2 Molecular Constants (in cm01) for the K1P State of Y35Cl
Te(£, J) Å Te / [ve 0 vexe(£ / 1/2)](£ / 1/2) / [Be 0 ae(£ / 1/2)]J(J / 1) 2
[2] 2
0 [De / be(£ / 1/2)]J (J / 1)
and the f levels of K1P by Tf(£, J) Å Te(£, J) / qJ(J / 1) 0 qDJ2(J / 1)2,
[3]
Copyright q 1996 by Academic Press, Inc.
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219
THE K1P –X1S/ TRANSITION OF YCl
TABLE 3 A Comparison between the ab Initio Study and the Experiments—Molecular Constants for the 1P States
Note. All values are given in cm01 except the re values which are given ˚. in A
yttrium 5s/5p hybrid, mainly localized on the yttrium atom, while the 3p is mostly the yttrium 5pp orbital and should be less antibonding than the 2p orbital. Comparing the rotational constants of the J1P and K1P states (Table 3), the Be value of the K1P state is smaller (the bond length re is accordingly longer) than that of the J1P state, which suggests that the electron configuration responsible for the K1P state has more antibonding character. In this MO picture, the 3s2p seems to be the most antibonding configuration. It is highly possible that the K1P state (Te Å 30 401 cm01) has the dominant configuration 3s2p, while in the J1P state, 2s3p should be the leading configuration instead of 3s2p, which Langhoff et al. suggested. As can be seen, both the J1P –X1S/ and K1P –X1S/ transitions contain large proportions of the 5p–5s character of the Y/ atomic transition.
The L-type doubling in the K1P state is surprisingly large compared with other 1P states (B1P, D1P, and J1P), about one order of magnitude larger than the L-type doubling in the B1P and D1P states; see Table 3. The negative value of the L-type doubling constant q for the K1P state shows that the f levels lie below the e levels. These two factors suggest that there is either a close-lying 1S/ state just below the K1P state or a 1S0 state just above the K1P state, which can originate for example from 2s4s (1S/), 2p2 (1S/), or 2p3p (1S/ and 1S0), respectively. REFERENCES 1. G. M. Janney, J. Opt. Soc. Am. 56, 1706–1711 (1966). 2. K. P. Huber and G. Herzberg, ‘‘Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules,’’ pp. 672–673, Van Nostrand–Reinhold, New York, 1979. 3. D. R. Fischell, H. C. Brayman, and T. A. Cool, J. Chem. Phys. 73, 4260–4272 (1980). 4. S. R. Langhoff, C. W. Bauschlicher, Jr., and H. Partridge, J. Chem. Phys. 89, 396–407 (1988). 5. R. Gopal, L. K. Singh, and M. M. Joshi, J. Mol. Spectrosc. 89, 15–18 (1981). 6. J. Xin, G. Edvinsson, and L. Klynning, J. Mol. Spectrosc. 148, 59–64 (1991). 7. J. Xin, G. Edvinsson, and L. Klynning, Phys. Scr. 47, 75–81 (1993). 8. J. Xin, G. Edvinsson, L. Klynning, and P. Royen, J. Mol. Spectrosc. 158, 14–20 (1993). 9. B. Simard, A. M. James, and P. A. Hackett, J. Chem. Phys. 96(4), 2565–2572 (1992). 10. K. D. Hensel and M. C. L. Gerry, J. Mol. Spectrosc. 166, 304–309 (1994). 11. B. A. Palmer and R. Engleman, Jr., ‘‘Atlas of the Thorium Spectrum,’’ Los Alamos, New Mexico (unpublished).
Copyright q 1996 by Academic Press, Inc.
AID
JMS 6922
/
m4652$$$90
01-18-96 00:53:31
mspa
AP: Mol Spec