- Email: [email protected]
The ground-state infrared spectrum of strontium monohydride (SrH)
Volume 15 I, number
3
CHEMICAL
THE GROUND-STATE
INFRARED
PHYSICS LETTERS
SPECTRUM
14Octobcr
OF STRONTIUM
MONOHYDRIDE
1988
(SrH)
Ulrich MAGG, Helmut BIRK and Harold JONES Abieilung Physikahsche
Chemie, Umversit& Urn, D-7900 Urn, FederalRepublic
qfCermany
Received 20 July 1988
The gas-phase Infrared spectrum of the main isotopic form of strontium monohydride, “SrH in natural abundance (82%), in the ground electronic state (IX’) has been observed using a diode laser spectrometer. Coupling between the unpaired electron and the overall rotation results in each individual transition being split into a doublet. The wavenumbers of eleven transitions of the v= I +O band, eight transitions of the Y= 2+ 1 band and eight transitions of the ~~3-2 band have been measured with a nominal accuracy of -tO.OOl cm-‘. In all cases the spin-rotational splitting (y splitting), which lay between 0.16 and 0.05 cm-‘, was easilyresolved and we were able to determine higher-order correction terms.
1. Introduction Diode laser spectroscopy of the ground state of monohydrides of several of the metals of group II of the periodic table have recently been reported. This includes the work on MgH and CaH by Lemoine et al. [ 11 and work from the present authors on BaH [ 21. In this paper we report a similar study on SrH. Strontium monohydride has been the object of spectroscopic investigation since the early thirties, see e.g. ref. [ 31. The best ground-state data so far published are undoubtedly those of Appelblad et al. [4], who studied the A-X and B-X band systems using Fourier transform spectroscopy. The upper state of both these bands were perturbed. It seemed, therefore, quite likely that the parameters determined in the case of SrH were not as reliable as the extremely accurate values obtained by the same authors for BaH [ 5 1. We were convinced that a direct investigation of the ground state with the diode laser could produce improved information on the ground electronic state of SrH.
2. Experimental The best yields of BaH were produced by simply heating barium metal to 1000°C in a hydrogen atmosphere in a stainless-steel tube [2]. However, 0 009-2614/88/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
presumably due to the higher stability of SrH, compared to that of BaH,, this approach failed to produce SrH. The technique used in this laboratory for CsH [ 6 ] proved more successful. The cell used consisted of an aluminium oxide ceramic tube 120 cm long and 20 mm in diameter. Two cylindrical, water-cooled stainless-steel electrodes were fitted co-axially to the ends of the tube and glass extensions were attached to the outer ends of the electrodes. Each extension carried a side arm to serve as gas inlet and outlet and were closed with KBr Brewster windows. The cell was charged with approximately 5 g of strontium and was placed within a tube furnace. After evacuation, the cell was filled with a 2 : 1 mixture of hydrogen and helium to a total pressure of about 6 mbar and heated to 860°C. The gas pressure was maintained at the same total pressure. The gas was discharged using the output of a 6.5 kV transformer and optimum conditions were obtained using a discharge current of 300 mA. A single charge of strontium allowed up to 16 h of measurement. The diode laser spectrometer used was based on the laser head assembly of Spectra Physics with diodes from the same company. Measurements were carried out to a nominal accuracy of 0.00 1 cm-’ using a calibrated confocal etalon with a FSR of 0.0098 11 cm- ’in conjunction with accurately measured absorption lines. The spectral range searched B.V.
263
Volume
15I, number 3
CHEMICAL
PHYSICS LETTERS
extended from 1040 to 1200 cm- ‘. Absolute wavenumber calibration was carried out using accurately known absorption lines of SOz [ 7 1. The diode laser beam was passed axially through the cell onto a HgCdTe infrared detector. Signals were processed by source modulation of the diode laser at 8 kHz followed by phase sensitive detection.
3. Spectra and analysis An example of the absorption signals observed is shown in fig. 1. As can be seen, the two ro-vibrational transitions shown, P(4), U= 1 t0 and R(5), v=3+2, appear as doublets separated by approximately 0.1 cm-‘. The splitting is due to the coupling between the overall rotation and the electron spin. The data obtained are shown with their assignments in table 1. Since the ground electronic state of SrH is ‘C, the usual Dunham [ 8 ] expression for the energy levels of a diatomic molecule F=C
Y,,(v+;)‘[N(N+l)]’ u
has to be extended to include terms describing spin splitting. These take the form
the
14 October
I98 8
forJ=N+f,
for J=N-f. The residuals of the tit to these expressions are shown in parentheses in table 1, the parameters fitted and the values obtained are shown in the first column of table 2.
4. Discussion The ground state parameters determined by Appelblad ct al. [ 41 arc also listed in table 2. Direct comparison is somewhat difficult since these authors chose to fit their data to a different parameter set ((I),-, WJ,, etc.). However, in a number of cases the corrections are so small that this can be done without introduction of too much uncertainty. As can be seen from this table, the pure vibrational terms, w,, ou,, determined by Appelblad et al. [4] differ considerably from those ( Ylo, Y,,) of the present work. This
P( 4) ” = I+0 I
R(51
1142.5
1142.6
1142.7
1142.8
1142.9
cm
-1
Fig. 1. The spectrum of ‘*SrH near 1142.5 cm-‘. The strong doublet separated by 0.13 cm-’ arises from the P(4) transition of the fundamental. The weaker doublet, to lower frequency, is the R (5) transition ofthe v= 3-2 band. Although lines from both *7SrH (7.0%) and HbSrH (9.8%) were also expected, the signals were generally too weak to be observed.
264
Volume
I 51, number 3
Table I Observed
infrared
CHEMICAL
transitions
of %H
(cm-‘)
PHYSICS LETTERS
14 October
I98 8
a’
v=l+O P(3) P(4) P(7)
1150.4691(00) 1142.7403( 13) 111X.6767(-04)
1150.6037(06) 1142.8788( 19) 11 1X.8257(-03)
P(8) P(9) P(l1)
1110.3763(-02) 1101.9400(13) 1084.6805(11)
1110.5286(-03) 1102.0962(-09) 1084.8427(07)
1101.5038(03) 1069.2085(01)
106X.0748(07) 1060.3634(-17)
R(1) R(5) R(6) R(8) R(13)
1193.57lS(Ol) 1212.8698(00) 1218.9469( -04) 1230.5500( -03) 1256.1906(03)
1193.6814(04) 1212.9656( -07) 1219.0384(-09) 1230.6317(-13) 1256.2493(06)
1101.6411(01) 1069.3603(01)
R( 3) R(4) R(4) R(l0) R( 11) R( 13)
1165.3764( -02) 1171.6477(08) 1183.6528(02) 1205.4341(-13) 1210.3980( -05) 1219.7244(08)
1165.4776( -03) 1171.7443(05) 1183.7407(02) 1205.5037(-09) 1210.4621(-07) 1219.7788(08)
1068.2080(04) 1060.5004(19)
R(5) R(6) R(lO) R( 11) R(l3) R( 15)
1142.5285(21) 1148.2676(-25) 1169.3673(11) 1174.1552(05)
1142.6168(20) 1148.3535( -05) 1169.4316(03) 1174.2153(03) 1183.1778(00) 1191.3140(-02)
v=2el P(5) P(9)
v=3+2 P(5) P(6)
‘I Numbers
in parentheses
represent
the deviation
This work YI” Yzu 102Y30 Y I&, , lf15YL, lO’Y,, 1o4 Y02 10hY,*. 109r,1, Yo1 10% I 1OSYOL al Numbers
for strontium
hydride
(cm~ ‘) a)
Ref. [4]
1206.8912(15) -17.02566(93) -1.162(15) 3.673447(22) -8.0132(16) 2.23(91) -4.02( 15) 1.3505( 14) 0 h, 3.03( 19) 0.12622(42) -4.062(38) -1.16(16) in parentheses
w, w&?
1207.040(3) 17.1083(11)
B, lO%Y,
3.673530(20) 8.0232(6)
104D,
1.35337(28)
109H, )‘c IO’& lOQ& represent
units of the last digit. ‘I Not determinable (constrained
3.577(13) 0.1259(9) -4.10(4) l-110(6)
one standard
at this value).
-06)
in units of the last digit.
is presumably caused by a systematic shift introduced via the perturbation. Most of the other terms are in good agreement with the present work. Our Table 2 Dunham pal-ameters determined
1183.1274(-01)
1191.2736(
deviation
in
measurements have allowed a number of higher-order parameters to be determined for the first time. Comparison of the Dunham parameters of table 2 with those determined for BaH [Z] reveals several differences. Y,, has the opposite sign in the two cases (SrH=-O.O116(1)cm-‘,BaH=O.O279(1)cm-’) and Y3, was determinable for SrH but not for BaH, the reverse being the case for YIr. Using the parameters of table 2, the coefficients in the potential function
where t= (r-r,) /re and B,= hl Wpr:, can be evaluated [ 8 1. The values obtained are shown in table 3. At the beginning of this work we had expected to observe transitions from the two less abundant isotopic species *‘SrH (7.0%) and ?$rH (9.8%)) as was the case with BaH [ 2 1. However, as can be seen from fig. 1, signals from this source were not obvious. Calculations showed that transitions from these isotopic 265
Volume Table 3 Potential
I5 1, number 3
constants
CHEMICAL
PHYSICS LETTERS
99920(102) -2.1992(77) 2.867(21) -2X(13) 2.1460967(74)
References
forms should have been easily resolvable. No particular effort was made to increase sensitivity in order to detect these lines and it would appear that the signal strength achieved for SrH was so much lower than that attained for BaH that this made observation difficult. This is probably due to SrH being relatively less thermodynamically stable than BaH.
Acknowledgement
266
by the Dcutschc
1988
schungsgemeinschaft. The authors wish to thank Mr. R.D. Urban for his help with some of the measurements.
of %H
This work is supported
14 October
For-
1I] B. Lemoine, C. Demuynck, J.L. Destombesand P.B. Davies, private communication. [2] U. Magg, H. Birk and H. Jones, Chem. Phys. l,etters 149
(1988) 321. [3] W.W. Watson and W.R. Fricdrickson, 765. [4] 0. Appleblad, L.E. Berg, L. Klynning Physica Scripta 33 (1986) 415.
Phys. Rev. 39 (1932) and J.W.C. Johns,
[5] 0. Appleblad, L.E. Berg, L. KIynning and J.W.C. Johns, Physica Scripta 3 I ( 1985 ) 69. [ 61 U. Magg and I-I. Jones, Chem. Phys. Letters 148 ( 1988) 6. [ 71 G. Guelachvill and K.N. Kao. Handbook of Infrared standards (Academic Press, New York. 1986). [8] J.L. Dunham. Phys. Rev. 41 (1932) 721.
3
CHEMICAL
THE GROUND-STATE
INFRARED
PHYSICS LETTERS
SPECTRUM
14Octobcr
OF STRONTIUM
MONOHYDRIDE
1988
(SrH)
Ulrich MAGG, Helmut BIRK and Harold JONES Abieilung Physikahsche
Chemie, Umversit& Urn, D-7900 Urn, FederalRepublic
qfCermany
Received 20 July 1988
The gas-phase Infrared spectrum of the main isotopic form of strontium monohydride, “SrH in natural abundance (82%), in the ground electronic state (IX’) has been observed using a diode laser spectrometer. Coupling between the unpaired electron and the overall rotation results in each individual transition being split into a doublet. The wavenumbers of eleven transitions of the v= I +O band, eight transitions of the Y= 2+ 1 band and eight transitions of the ~~3-2 band have been measured with a nominal accuracy of -tO.OOl cm-‘. In all cases the spin-rotational splitting (y splitting), which lay between 0.16 and 0.05 cm-‘, was easilyresolved and we were able to determine higher-order correction terms.
1. Introduction Diode laser spectroscopy of the ground state of monohydrides of several of the metals of group II of the periodic table have recently been reported. This includes the work on MgH and CaH by Lemoine et al. [ 11 and work from the present authors on BaH [ 21. In this paper we report a similar study on SrH. Strontium monohydride has been the object of spectroscopic investigation since the early thirties, see e.g. ref. [ 31. The best ground-state data so far published are undoubtedly those of Appelblad et al. [4], who studied the A-X and B-X band systems using Fourier transform spectroscopy. The upper state of both these bands were perturbed. It seemed, therefore, quite likely that the parameters determined in the case of SrH were not as reliable as the extremely accurate values obtained by the same authors for BaH [ 5 1. We were convinced that a direct investigation of the ground state with the diode laser could produce improved information on the ground electronic state of SrH.
2. Experimental The best yields of BaH were produced by simply heating barium metal to 1000°C in a hydrogen atmosphere in a stainless-steel tube [2]. However, 0 009-2614/88/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
presumably due to the higher stability of SrH, compared to that of BaH,, this approach failed to produce SrH. The technique used in this laboratory for CsH [ 6 ] proved more successful. The cell used consisted of an aluminium oxide ceramic tube 120 cm long and 20 mm in diameter. Two cylindrical, water-cooled stainless-steel electrodes were fitted co-axially to the ends of the tube and glass extensions were attached to the outer ends of the electrodes. Each extension carried a side arm to serve as gas inlet and outlet and were closed with KBr Brewster windows. The cell was charged with approximately 5 g of strontium and was placed within a tube furnace. After evacuation, the cell was filled with a 2 : 1 mixture of hydrogen and helium to a total pressure of about 6 mbar and heated to 860°C. The gas pressure was maintained at the same total pressure. The gas was discharged using the output of a 6.5 kV transformer and optimum conditions were obtained using a discharge current of 300 mA. A single charge of strontium allowed up to 16 h of measurement. The diode laser spectrometer used was based on the laser head assembly of Spectra Physics with diodes from the same company. Measurements were carried out to a nominal accuracy of 0.00 1 cm-’ using a calibrated confocal etalon with a FSR of 0.0098 11 cm- ’in conjunction with accurately measured absorption lines. The spectral range searched B.V.
263
Volume
15I, number 3
CHEMICAL
PHYSICS LETTERS
extended from 1040 to 1200 cm- ‘. Absolute wavenumber calibration was carried out using accurately known absorption lines of SOz [ 7 1. The diode laser beam was passed axially through the cell onto a HgCdTe infrared detector. Signals were processed by source modulation of the diode laser at 8 kHz followed by phase sensitive detection.
3. Spectra and analysis An example of the absorption signals observed is shown in fig. 1. As can be seen, the two ro-vibrational transitions shown, P(4), U= 1 t0 and R(5), v=3+2, appear as doublets separated by approximately 0.1 cm-‘. The splitting is due to the coupling between the overall rotation and the electron spin. The data obtained are shown with their assignments in table 1. Since the ground electronic state of SrH is ‘C, the usual Dunham [ 8 ] expression for the energy levels of a diatomic molecule F=C
Y,,(v+;)‘[N(N+l)]’ u
has to be extended to include terms describing spin splitting. These take the form
the
14 October
I98 8
forJ=N+f,
for J=N-f. The residuals of the tit to these expressions are shown in parentheses in table 1, the parameters fitted and the values obtained are shown in the first column of table 2.
4. Discussion The ground state parameters determined by Appelblad ct al. [ 41 arc also listed in table 2. Direct comparison is somewhat difficult since these authors chose to fit their data to a different parameter set ((I),-, WJ,, etc.). However, in a number of cases the corrections are so small that this can be done without introduction of too much uncertainty. As can be seen from this table, the pure vibrational terms, w,, ou,, determined by Appelblad et al. [4] differ considerably from those ( Ylo, Y,,) of the present work. This
P( 4) ” = I+0 I
R(51
1142.5
1142.6
1142.7
1142.8
1142.9
cm
-1
Fig. 1. The spectrum of ‘*SrH near 1142.5 cm-‘. The strong doublet separated by 0.13 cm-’ arises from the P(4) transition of the fundamental. The weaker doublet, to lower frequency, is the R (5) transition ofthe v= 3-2 band. Although lines from both *7SrH (7.0%) and HbSrH (9.8%) were also expected, the signals were generally too weak to be observed.
264
Volume
I 51, number 3
Table I Observed
infrared
CHEMICAL
transitions
of %H
(cm-‘)
PHYSICS LETTERS
14 October
I98 8
a’
v=l+O P(3) P(4) P(7)
1150.4691(00) 1142.7403( 13) 111X.6767(-04)
1150.6037(06) 1142.8788( 19) 11 1X.8257(-03)
P(8) P(9) P(l1)
1110.3763(-02) 1101.9400(13) 1084.6805(11)
1110.5286(-03) 1102.0962(-09) 1084.8427(07)
1101.5038(03) 1069.2085(01)
106X.0748(07) 1060.3634(-17)
R(1) R(5) R(6) R(8) R(13)
1193.57lS(Ol) 1212.8698(00) 1218.9469( -04) 1230.5500( -03) 1256.1906(03)
1193.6814(04) 1212.9656( -07) 1219.0384(-09) 1230.6317(-13) 1256.2493(06)
1101.6411(01) 1069.3603(01)
R( 3) R(4) R(4) R(l0) R( 11) R( 13)
1165.3764( -02) 1171.6477(08) 1183.6528(02) 1205.4341(-13) 1210.3980( -05) 1219.7244(08)
1165.4776( -03) 1171.7443(05) 1183.7407(02) 1205.5037(-09) 1210.4621(-07) 1219.7788(08)
1068.2080(04) 1060.5004(19)
R(5) R(6) R(lO) R( 11) R(l3) R( 15)
1142.5285(21) 1148.2676(-25) 1169.3673(11) 1174.1552(05)
1142.6168(20) 1148.3535( -05) 1169.4316(03) 1174.2153(03) 1183.1778(00) 1191.3140(-02)
v=2el P(5) P(9)
v=3+2 P(5) P(6)
‘I Numbers
in parentheses
represent
the deviation
This work YI” Yzu 102Y30 Y I&, , lf15YL, lO’Y,, 1o4 Y02 10hY,*. 109r,1, Yo1 10% I 1OSYOL al Numbers
for strontium
hydride
(cm~ ‘) a)
Ref. [4]
1206.8912(15) -17.02566(93) -1.162(15) 3.673447(22) -8.0132(16) 2.23(91) -4.02( 15) 1.3505( 14) 0 h, 3.03( 19) 0.12622(42) -4.062(38) -1.16(16) in parentheses
w, w&?
1207.040(3) 17.1083(11)
B, lO%Y,
3.673530(20) 8.0232(6)
104D,
1.35337(28)
109H, )‘c IO’& lOQ& represent
units of the last digit. ‘I Not determinable (constrained
3.577(13) 0.1259(9) -4.10(4) l-110(6)
one standard
at this value).
-06)
in units of the last digit.
is presumably caused by a systematic shift introduced via the perturbation. Most of the other terms are in good agreement with the present work. Our Table 2 Dunham pal-ameters determined
1183.1274(-01)
1191.2736(
deviation
in
measurements have allowed a number of higher-order parameters to be determined for the first time. Comparison of the Dunham parameters of table 2 with those determined for BaH [Z] reveals several differences. Y,, has the opposite sign in the two cases (SrH=-O.O116(1)cm-‘,BaH=O.O279(1)cm-’) and Y3, was determinable for SrH but not for BaH, the reverse being the case for YIr. Using the parameters of table 2, the coefficients in the potential function
where t= (r-r,) /re and B,= hl Wpr:, can be evaluated [ 8 1. The values obtained are shown in table 3. At the beginning of this work we had expected to observe transitions from the two less abundant isotopic species *‘SrH (7.0%) and ?$rH (9.8%)) as was the case with BaH [ 2 1. However, as can be seen from fig. 1, signals from this source were not obvious. Calculations showed that transitions from these isotopic 265
Volume Table 3 Potential
I5 1, number 3
constants
CHEMICAL
PHYSICS LETTERS
99920(102) -2.1992(77) 2.867(21) -2X(13) 2.1460967(74)
References
forms should have been easily resolvable. No particular effort was made to increase sensitivity in order to detect these lines and it would appear that the signal strength achieved for SrH was so much lower than that attained for BaH that this made observation difficult. This is probably due to SrH being relatively less thermodynamically stable than BaH.
Acknowledgement
266
by the Dcutschc
1988
schungsgemeinschaft. The authors wish to thank Mr. R.D. Urban for his help with some of the measurements.
of %H
This work is supported
14 October
For-
1I] B. Lemoine, C. Demuynck, J.L. Destombesand P.B. Davies, private communication. [2] U. Magg, H. Birk and H. Jones, Chem. Phys. l,etters 149
(1988) 321. [3] W.W. Watson and W.R. Fricdrickson, 765. [4] 0. Appleblad, L.E. Berg, L. Klynning Physica Scripta 33 (1986) 415.
Phys. Rev. 39 (1932) and J.W.C. Johns,
[5] 0. Appleblad, L.E. Berg, L. KIynning and J.W.C. Johns, Physica Scripta 3 I ( 1985 ) 69. [ 61 U. Magg and I-I. Jones, Chem. Phys. Letters 148 ( 1988) 6. [ 71 G. Guelachvill and K.N. Kao. Handbook of Infrared standards (Academic Press, New York. 1986). [8] J.L. Dunham. Phys. Rev. 41 (1932) 721.