- Email: [email protected]
Laser excitation spectroscopy of the B2∑ - X2∑ transition of the CaH molecule
LASER EXCITATION
June 1976
OPTICS COMMUNICATIONS
Volume 17, number 3
SPECTROSCOPY
OF THE B 2 Z - X 2 Z TRANSITION
OF THE CaH MOLECULE
L.E. BERG, L. KLYNNING
and H. MARTIN
Institute of Physics, University of Stockholm,
Vanadisv. 9, S-l 13 46 Stockholm, Sweden
Received 2 February 1976
The B ‘Z state of CaH has been examined by laser excitation spectroscopy with a cw dye laser. The excitation spectra were obtained by tuning the frequency of the dye laser, and the fluorescence was detected by using a scanning monochromator. Wavelengths of the CaH band lines were determined relative to interference fringes of a confocal Fabry-Perot interferometer, which in turn were calibrated against known absorption lines from iodine. To new bands belonging to the B-X band system of CaH have been recorded. These bands were too weak to be seen by ordinary absorption spectroscopy, but are easily detected with the present technique. An ambiguity in the earlier analysis of the u’ = 2 state, due to a strong perturbation by the A ‘n state at low J-values, has been removed by the new fluorescence data.
1. Introduction
Many experiments and calculations of the CaH molecule have been performed in the past, largely due to its astrophysical interest [l-4]. The X * Z ground state has been thoroughly examined, and the molecular constants for this state have been calculated [4]. The excited states A 2 Il and B 2 t: perturb each other. Perturbation calculations have been performed on the A 2 Il(u = O)B 2 C(u = 0) system. The future success of a complete deperturbation depends on precise information on the higher vibrational levels, especially at low J-values, where the interaction between the A and B states is strong. The Au = 0 sequences of both the A-X and B-X states consist of close lying bands that dominate the spectrum and causes severe overlaps. Moreover the Au = 0 sequence of the B-X system almost coincides with the Av = 1 sequence of the A-X system, a complication which makes the analysis of the weaker bands and, in particular, the perturbed regions even more uncertain. Here, the high selectivity of certain laser techniques offers new means to get reliable assignments. In the wavelength region 5800-6050 A, one expects to find CaH bands belonging to the Au = 1 and Au = 2 sequences of the B-X and A-X transitions respectively. However, no such bands were observed by ordinary absorption methods, even when the Au =0 se320
quences were saturated. Calculated Franck-Condon factors of these transitions showed that very little intensity indeed is to be expected in any of the sequences but Au = 0.
”
A2Tl
9*X
3
r y 2
2
LSI
0
I i-t-t0
DYE LASER EXCITATION
II1 I/
Ill
I
FLUORESCENCE
x 2z Fig. 1. Term level diagram of CaH showing the laser excitation transition and the fluorescence transition used the monitor the registration.
Volume 17, number 3
Using a tunable cw Rhodamine 6G dye laser, however, we have obtained excitation of the CaH molecules from the u = 0 and v = 1 levels of the X2 E ground state to the u = 1 and u = 2 levels of the B 2C state (see fig. 1) without undue difficulties. The corresponding excitation of the A211 state was not seen at all since here the FC factors are several orders of magnitude smaller.
2. Experimental
June 1976
OPTICS COMMUNICATIONS
procedure
In order to produce CaH, a King furnace was used. High temperature furnaces of the King type are capable of forming many molecules of astrophysical interest under controlled conditions but are normally not used in laser excitation spectroscopy. One reason is that in a long tube the observations must be made in axial direction and so special precautions have to be made in order to avoid direct and reflected laser light, as well as light from the heating tube. These difficulties can be overcome by quite simple means (see fig. 2): a narrow (2 mm) laser beam enters the furnace through a clear aperture in an aluminized mirror set at 45” to the tube axis. This mirror is sealed with an O-ring to the furnace and acts as end window. The mirror also reflects the radiated light coming backwards from the fluorescence region inside the tube to a telephoto lens and a
1 mm pinhole in front of a scanning monochromator. With proper focusing, the monochromator sees a volume of about only 20 cm long and 1 cm diameter, surrounding the fluorescence region. Almost no light from the walls of the graphite tube or laser light from the other end window could be detected. Since the laser light was plane polarized, the insertation of a crossed Nicole prism drastically decreased the intensity of the scattered laser light, whereas only half of the fluorescent radiation was lost. The furnace was run at a temperature of about 920-940°C and was held at a maximum pressure of 50 mm Hg when heated. These conditions are to be compared to the earlier absorption experiments of the red CaH bands [4] where much higher temperature and pressure were used, i.e. 1600°C and 600 mm Hg respectively. In order to excite the CaH molecule a continuouswave Rhodamine 6G jet-stream dye laser was used. An argon-ion laser giving a power of 5 W (all lines) was used to excite the dye, and both lasers were mounted on a vibration-isolated marble table. The dye laser was operated with one intra-cavity etalon (thickness 3 mm) giving about five longitudinal modes and thus restricting the resolution to about 0.05 cm-l. The output power of the dye laser was about SO--150 mW and was kept constant by a feed back loop to the Ar pump laser. The laser frequency was tuned by tilting the intracavity
to RECORDER D3
REFERENCE
from D3
to RECORDER
from FPS
Fig. 2. Schematic drawing of the experimental laser excitation spectroscopy setup. Dotted lines represent electric connections. D2 and D3 = photodiodes. The Dl-circuit shows the feed-back mechanism keeping the laser power constant.
Dl,
321
Volume 17, number 3
June 1976
OPTICS COMMUNICATIONS
Table 1 Wave numbers in cm-’ of the observed (1,O) and (2,l) band lines of the B% -X2x
system of CaH.
(291)
(l,O)
p2 0.5
16921.61
973.68
037.13
913.15
954.76
906.36
968.95 961.20
046.64 056.33 066.15
904.91 896.88
963.54 972.30
898.49 890.56
889.15 881.61
981.31
862.35
17000.72
16877.38
873.61 863.80
003.03 015.86
868.34 860.28
853.27 841.80
024.27 032.76
852.48
829.75
041.37
008.57
817.48
049.88
011.45 013.99
031.82 040.93
955.80
049.98
948.11 940.40
059.26 068.63
953.90 946.84
932.98 925.72
078.04 087.52
933.27 926.68
096.98
920.21
11.5
918.58 911.58
12.5 13.5
904.66 897.82
907.59 901.43
14.5
2.5 3.5 4.5 5.5 6.5 7.5 6.5 9.5
980.70 972.12 963.92
10.5
939.96
891.12 884.44
895.38 889.40
877.93
17.5 18.5
871:30 864.65
883.53 877.69 871.88
19.5
858.53
866.13
20.5
852.17
860.55
21.5
845.89
22.5
839.52
854.97 849.26
23.5
833.15 826.85
843.52
820.53 814.07
832.06
25.5 26.5 27.5 28.5
Notice the extra
076.09 086.05
874.15 866.69
096.06
913.82
15.5 16.5
24.5
990.26 999.16 17007.77
16947. 16914.12
973.02
016.39 024.82 033.05
845.02
836.98
041.42 C-IS.16 056.79
837.64
058.21
830.29 823.02
066.45 074.47 082.41
821.63 813.80
064.23 071.46
815.67
GS
956.84 964.89
859.41 851.97 844.50 829.27
930.84 987.39 993.03 997.77 17001.70 005.30
090.05
078.34 084.93
837.86 826.18 820.24 814.16
P2 and R2 lines of the (2,l) band.
etalon, and the maximum scan range was about 2.5 A. The laser was mechanically chopped to permit the use of a lock-in amplifier connected to the photomultiplier in the detection system. In certain cases, when highest possible intensity was needed, the mechanical chopper was replaced by a system electrically modulating the Ar-ion laser current. The fluorescence from the molecules that were excited by the laser beam was focussed on the entrance slit of a small scanning monochromator. By choosing a suitable band width of the monochromator and by matching the scan rates of the monochromator and the dye laser, the output signal proved to be a very sensitive indicator of the primary absorption from the selected band free from any interference from other bands. 322
R2
028.14
16982.03
16989.56
p2 16937.93 946.26
17006.12 014.44 023.10
1.5
R2 17015.96
The laser beam was split and sent to a spherical mirror Fabry-Perot interferometer registering the frequency changes when scanning the dye laser. The interferometer was placed in a room having a constant temperature in order to avoid temperature changes affecting it. The free spectral range was 8 GHz. In order to get a wavelength calibration, an iodine cell was set up so that absorption spectra could be recorded simultaneously with the excitation spectra. An ordinary photodiode was placed behind the iodine cell and connected to the recorder. The calibration of the iodine spectrum was made by recording the spectrum photographically in a 10.7 m grating spectrograph. The spectrograms were measured and wavenumbers were determined by standard methods. The wavelengths of the iron reference lines were obtained from the Crosswhite tables [5].
Volume 17, number 3
OPTICS COMMUNICATIONS
Fig. 3. At the top is shown a recording trace of the (1,O) and (2,l) bands of the B2C -X% shows the FPS-fringes; the bottom one shows the iodine absorption lines.
With this laser excitation technique, we have observed and measured the (1,O) and (2,l) bands of the B-X transition covering the wavelength region 5850 to 5960 A. Since the scanning interval is a 15 to 20 A wide interval in a typical single run (required about 20 minutes of observation), several runs were needed. This situation is a frequent one in molecular spectroscopy and one has to compromise between large scan ranges and spectral resolution. High resolving power for instance is possible only in single mode operation of the laser, but then, the attainable scan range is less than la. The wavenumbers of the observed lines (see table 1) were determined for each run separately, although it was found that with a temperature stabilized FPS, the method of adding wavenumber differences would have given the same results. The observed wavenumbers of the (1,O) bandlines agree very well with the ones calculated from earlier data, whereas for the (2,1) band it was necessary to increase the numbering of the RI branch of the earlier given (2,2) band by one unit and to alter the corresponding PI branch, which was masked by the,much stronger (0,O) and (1,l) band heads. New term values for the complete set of lines were determined and resulted in revised values for the u’= 2 level. A number of extra term values in the perturbed regions were observed. The top recording trace in fig. 3 shows part of the (1,O) and (2,1) bands of the B-X system registered in a single run with laser excitation spectroscopy. Notice in the (2,l) band of the B-X system the extra
June 1976
transition of CaH. The middle trace
A-X band lines indicating the perturbation between the A and B systems. The middle trace shows a recording of the interference fringes separated by Au = 0.267 cm-l, and the bottom one shows the iodine absorption spectrum.
3. Summary In this paper a laser excitation spectroscopy method with several advantageous features is described and compared to conventional absorption spectroscopy. Firstly, it is shown to be a very sensitive way to record band spectra. Two new bands in CaH have been discovered and not even the strongest lines in these bands have been observed by ordinary photographic techniques. Secondly, due to the fact that the absorption lines are monitored by the fluorescence from the excited state, one has the possibility of recording molecular bands selectively. Moreover, by operating the laser in a multimode configuration, which means that the extreme resolving power has to be sacrificed to some extent, a relatively wide scan range has been obtained, a fact which is of great importance in molecular spectroscopy. The resolution is still comparable to that of photographic registration in high resolution spectrographs and could be further improved by using intermodulated fluorescence technique to remove doppler broadening. The success with the present rather unusual optical arrangement proves that one can make sensitive laser 323
Volume 17, number 3
OPTICS COMMUNICATIONS
fluorescence spectroscopy also when viewing parallel1 to the incoming laser beam. This possibility permits use of large high temperature furnaces. The King furnace used in these experiments give in its present design a maximum temperature of about 3300°C [6]. High temperature can, of course, be of great importance when forming molecules of astrophysical interest. The interpretation of our data, including detailed analysis of the perturbations in terms of model fitting, is still in progress.
324
June 1976
Acknowledgments We wish to express our gratitude to Prof. A. Lagerqvist for his keen interest in this work.
References [l] E. Hulthdn, Phys. Rev. 29 (1927) 97. [2] W.W.Watson, and R.L. Weber, Phys. Rev. 48 (1935) 732. [3] G. Liberale and S. Weniger, Physica 41 (1969) 47. [4] L.-E. Berg, L. Klynning, Physica Scripta 10 (1974) 331. (51 H.M. Crosswhite, J. Research N.B.S. 79A (1975) 17. [6] B. GrundstrGm, Thesis, Stockholm (1936).
June 1976
OPTICS COMMUNICATIONS
Volume 17, number 3
SPECTROSCOPY
OF THE B 2 Z - X 2 Z TRANSITION
OF THE CaH MOLECULE
L.E. BERG, L. KLYNNING
and H. MARTIN
Institute of Physics, University of Stockholm,
Vanadisv. 9, S-l 13 46 Stockholm, Sweden
Received 2 February 1976
The B ‘Z state of CaH has been examined by laser excitation spectroscopy with a cw dye laser. The excitation spectra were obtained by tuning the frequency of the dye laser, and the fluorescence was detected by using a scanning monochromator. Wavelengths of the CaH band lines were determined relative to interference fringes of a confocal Fabry-Perot interferometer, which in turn were calibrated against known absorption lines from iodine. To new bands belonging to the B-X band system of CaH have been recorded. These bands were too weak to be seen by ordinary absorption spectroscopy, but are easily detected with the present technique. An ambiguity in the earlier analysis of the u’ = 2 state, due to a strong perturbation by the A ‘n state at low J-values, has been removed by the new fluorescence data.
1. Introduction
Many experiments and calculations of the CaH molecule have been performed in the past, largely due to its astrophysical interest [l-4]. The X * Z ground state has been thoroughly examined, and the molecular constants for this state have been calculated [4]. The excited states A 2 Il and B 2 t: perturb each other. Perturbation calculations have been performed on the A 2 Il(u = O)B 2 C(u = 0) system. The future success of a complete deperturbation depends on precise information on the higher vibrational levels, especially at low J-values, where the interaction between the A and B states is strong. The Au = 0 sequences of both the A-X and B-X states consist of close lying bands that dominate the spectrum and causes severe overlaps. Moreover the Au = 0 sequence of the B-X system almost coincides with the Av = 1 sequence of the A-X system, a complication which makes the analysis of the weaker bands and, in particular, the perturbed regions even more uncertain. Here, the high selectivity of certain laser techniques offers new means to get reliable assignments. In the wavelength region 5800-6050 A, one expects to find CaH bands belonging to the Au = 1 and Au = 2 sequences of the B-X and A-X transitions respectively. However, no such bands were observed by ordinary absorption methods, even when the Au =0 se320
quences were saturated. Calculated Franck-Condon factors of these transitions showed that very little intensity indeed is to be expected in any of the sequences but Au = 0.
”
A2Tl
9*X
3
r y 2
2
LSI
0
I i-t-t0
DYE LASER EXCITATION
II1 I/
Ill
I
FLUORESCENCE
x 2z Fig. 1. Term level diagram of CaH showing the laser excitation transition and the fluorescence transition used the monitor the registration.
Volume 17, number 3
Using a tunable cw Rhodamine 6G dye laser, however, we have obtained excitation of the CaH molecules from the u = 0 and v = 1 levels of the X2 E ground state to the u = 1 and u = 2 levels of the B 2C state (see fig. 1) without undue difficulties. The corresponding excitation of the A211 state was not seen at all since here the FC factors are several orders of magnitude smaller.
2. Experimental
June 1976
OPTICS COMMUNICATIONS
procedure
In order to produce CaH, a King furnace was used. High temperature furnaces of the King type are capable of forming many molecules of astrophysical interest under controlled conditions but are normally not used in laser excitation spectroscopy. One reason is that in a long tube the observations must be made in axial direction and so special precautions have to be made in order to avoid direct and reflected laser light, as well as light from the heating tube. These difficulties can be overcome by quite simple means (see fig. 2): a narrow (2 mm) laser beam enters the furnace through a clear aperture in an aluminized mirror set at 45” to the tube axis. This mirror is sealed with an O-ring to the furnace and acts as end window. The mirror also reflects the radiated light coming backwards from the fluorescence region inside the tube to a telephoto lens and a
1 mm pinhole in front of a scanning monochromator. With proper focusing, the monochromator sees a volume of about only 20 cm long and 1 cm diameter, surrounding the fluorescence region. Almost no light from the walls of the graphite tube or laser light from the other end window could be detected. Since the laser light was plane polarized, the insertation of a crossed Nicole prism drastically decreased the intensity of the scattered laser light, whereas only half of the fluorescent radiation was lost. The furnace was run at a temperature of about 920-940°C and was held at a maximum pressure of 50 mm Hg when heated. These conditions are to be compared to the earlier absorption experiments of the red CaH bands [4] where much higher temperature and pressure were used, i.e. 1600°C and 600 mm Hg respectively. In order to excite the CaH molecule a continuouswave Rhodamine 6G jet-stream dye laser was used. An argon-ion laser giving a power of 5 W (all lines) was used to excite the dye, and both lasers were mounted on a vibration-isolated marble table. The dye laser was operated with one intra-cavity etalon (thickness 3 mm) giving about five longitudinal modes and thus restricting the resolution to about 0.05 cm-l. The output power of the dye laser was about SO--150 mW and was kept constant by a feed back loop to the Ar pump laser. The laser frequency was tuned by tilting the intracavity
to RECORDER D3
REFERENCE
from D3
to RECORDER
from FPS
Fig. 2. Schematic drawing of the experimental laser excitation spectroscopy setup. Dotted lines represent electric connections. D2 and D3 = photodiodes. The Dl-circuit shows the feed-back mechanism keeping the laser power constant.
Dl,
321
Volume 17, number 3
June 1976
OPTICS COMMUNICATIONS
Table 1 Wave numbers in cm-’ of the observed (1,O) and (2,l) band lines of the B% -X2x
system of CaH.
(291)
(l,O)
p2 0.5
16921.61
973.68
037.13
913.15
954.76
906.36
968.95 961.20
046.64 056.33 066.15
904.91 896.88
963.54 972.30
898.49 890.56
889.15 881.61
981.31
862.35
17000.72
16877.38
873.61 863.80
003.03 015.86
868.34 860.28
853.27 841.80
024.27 032.76
852.48
829.75
041.37
008.57
817.48
049.88
011.45 013.99
031.82 040.93
955.80
049.98
948.11 940.40
059.26 068.63
953.90 946.84
932.98 925.72
078.04 087.52
933.27 926.68
096.98
920.21
11.5
918.58 911.58
12.5 13.5
904.66 897.82
907.59 901.43
14.5
2.5 3.5 4.5 5.5 6.5 7.5 6.5 9.5
980.70 972.12 963.92
10.5
939.96
891.12 884.44
895.38 889.40
877.93
17.5 18.5
871:30 864.65
883.53 877.69 871.88
19.5
858.53
866.13
20.5
852.17
860.55
21.5
845.89
22.5
839.52
854.97 849.26
23.5
833.15 826.85
843.52
820.53 814.07
832.06
25.5 26.5 27.5 28.5
Notice the extra
076.09 086.05
874.15 866.69
096.06
913.82
15.5 16.5
24.5
990.26 999.16 17007.77
16947. 16914.12
973.02
016.39 024.82 033.05
845.02
836.98
041.42 C-IS.16 056.79
837.64
058.21
830.29 823.02
066.45 074.47 082.41
821.63 813.80
064.23 071.46
815.67
GS
956.84 964.89
859.41 851.97 844.50 829.27
930.84 987.39 993.03 997.77 17001.70 005.30
090.05
078.34 084.93
837.86 826.18 820.24 814.16
P2 and R2 lines of the (2,l) band.
etalon, and the maximum scan range was about 2.5 A. The laser was mechanically chopped to permit the use of a lock-in amplifier connected to the photomultiplier in the detection system. In certain cases, when highest possible intensity was needed, the mechanical chopper was replaced by a system electrically modulating the Ar-ion laser current. The fluorescence from the molecules that were excited by the laser beam was focussed on the entrance slit of a small scanning monochromator. By choosing a suitable band width of the monochromator and by matching the scan rates of the monochromator and the dye laser, the output signal proved to be a very sensitive indicator of the primary absorption from the selected band free from any interference from other bands. 322
R2
028.14
16982.03
16989.56
p2 16937.93 946.26
17006.12 014.44 023.10
1.5
R2 17015.96
The laser beam was split and sent to a spherical mirror Fabry-Perot interferometer registering the frequency changes when scanning the dye laser. The interferometer was placed in a room having a constant temperature in order to avoid temperature changes affecting it. The free spectral range was 8 GHz. In order to get a wavelength calibration, an iodine cell was set up so that absorption spectra could be recorded simultaneously with the excitation spectra. An ordinary photodiode was placed behind the iodine cell and connected to the recorder. The calibration of the iodine spectrum was made by recording the spectrum photographically in a 10.7 m grating spectrograph. The spectrograms were measured and wavenumbers were determined by standard methods. The wavelengths of the iron reference lines were obtained from the Crosswhite tables [5].
Volume 17, number 3
OPTICS COMMUNICATIONS
Fig. 3. At the top is shown a recording trace of the (1,O) and (2,l) bands of the B2C -X% shows the FPS-fringes; the bottom one shows the iodine absorption lines.
With this laser excitation technique, we have observed and measured the (1,O) and (2,l) bands of the B-X transition covering the wavelength region 5850 to 5960 A. Since the scanning interval is a 15 to 20 A wide interval in a typical single run (required about 20 minutes of observation), several runs were needed. This situation is a frequent one in molecular spectroscopy and one has to compromise between large scan ranges and spectral resolution. High resolving power for instance is possible only in single mode operation of the laser, but then, the attainable scan range is less than la. The wavenumbers of the observed lines (see table 1) were determined for each run separately, although it was found that with a temperature stabilized FPS, the method of adding wavenumber differences would have given the same results. The observed wavenumbers of the (1,O) bandlines agree very well with the ones calculated from earlier data, whereas for the (2,1) band it was necessary to increase the numbering of the RI branch of the earlier given (2,2) band by one unit and to alter the corresponding PI branch, which was masked by the,much stronger (0,O) and (1,l) band heads. New term values for the complete set of lines were determined and resulted in revised values for the u’= 2 level. A number of extra term values in the perturbed regions were observed. The top recording trace in fig. 3 shows part of the (1,O) and (2,1) bands of the B-X system registered in a single run with laser excitation spectroscopy. Notice in the (2,l) band of the B-X system the extra
June 1976
transition of CaH. The middle trace
A-X band lines indicating the perturbation between the A and B systems. The middle trace shows a recording of the interference fringes separated by Au = 0.267 cm-l, and the bottom one shows the iodine absorption spectrum.
3. Summary In this paper a laser excitation spectroscopy method with several advantageous features is described and compared to conventional absorption spectroscopy. Firstly, it is shown to be a very sensitive way to record band spectra. Two new bands in CaH have been discovered and not even the strongest lines in these bands have been observed by ordinary photographic techniques. Secondly, due to the fact that the absorption lines are monitored by the fluorescence from the excited state, one has the possibility of recording molecular bands selectively. Moreover, by operating the laser in a multimode configuration, which means that the extreme resolving power has to be sacrificed to some extent, a relatively wide scan range has been obtained, a fact which is of great importance in molecular spectroscopy. The resolution is still comparable to that of photographic registration in high resolution spectrographs and could be further improved by using intermodulated fluorescence technique to remove doppler broadening. The success with the present rather unusual optical arrangement proves that one can make sensitive laser 323
Volume 17, number 3
OPTICS COMMUNICATIONS
fluorescence spectroscopy also when viewing parallel1 to the incoming laser beam. This possibility permits use of large high temperature furnaces. The King furnace used in these experiments give in its present design a maximum temperature of about 3300°C [6]. High temperature can, of course, be of great importance when forming molecules of astrophysical interest. The interpretation of our data, including detailed analysis of the perturbations in terms of model fitting, is still in progress.
324
June 1976
Acknowledgments We wish to express our gratitude to Prof. A. Lagerqvist for his keen interest in this work.
References [l] E. Hulthdn, Phys. Rev. 29 (1927) 97. [2] W.W.Watson, and R.L. Weber, Phys. Rev. 48 (1935) 732. [3] G. Liberale and S. Weniger, Physica 41 (1969) 47. [4] L.-E. Berg, L. Klynning, Physica Scripta 10 (1974) 331. (51 H.M. Crosswhite, J. Research N.B.S. 79A (1975) 17. [6] B. GrundstrGm, Thesis, Stockholm (1936).