- Email: [email protected]
Time-resolved study of the A2Π state of CaH by laser spectroscopy
Journal of Molecular Spectroscopy 257 (2009) 105–107
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Note
Time-resolved study of the A2P state of CaH by laser spectroscopy M. Liu a, T. Pauchard a, M. Sjödin a, O. Launila a, P. van der Meulen b, L.-E. Berg a,* a b
Department of Applied Physics, KTH (Royal Institute of Technology), Roslagstullsbacken 21, S-106 91 Stockholm, Sweden Department of Physics, Stockholm University, S-106 91 Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 12 May 2009 In revised form 3 June 2009 Available online 13 June 2009 Keywords: Spectroscopy Life time Radicals Solar spectra
a b s t r a c t A time-resolved experiment on the A2P state of gaseous calcium hydride has been performed by applying laser spectroscopic methods. The following zero-pressure lifetime was obtained for the CaH A2P state: st=0 = 33.2 (±3.2) ns and st=1 = 33.7 (±5.2) ns. The lifetime was found to be the same for the A2P½ and A2P3/2 states. Ó 2009 Elsevier Inc. All rights reserved.
Gaseous CaH has been the subject of many experimental studies in the past. It is a molecule of great interest to chemists, physicists and of course to astrophysicists, due to its abundance in stellar atmospheres. Experiments to obtain electronic, vibrational and rotational constants as well as to perform lifetime measurements are thus of major importance. Bands of CaH and CaD have been observed in the solar spectrum and also in certain types of stars, where high abundances of these molecules have been registered [1]. Burgasser et al. have presented spectra for T dwarfs observed using the Keck I 10 m Telescope, where they observed a variety of features related to bands of CaH [2]. The chemical abundance of CaH in Kapteyn’s Star [3] has been studied by Woolf and Wallerstein. By applying a supersonic molecular beam, rotationally resolved spectra of the CaH radical have been reported. High-energy resolution of the (1 + 1’) REMPI spectra corresponding to the A2P–X2R transitions were measured, which allowed for the first time a clear and precise analysis of the low-rotational part [4]. The A2P–X2R (0,0) band of CaH has rather recently been shown to be a candidate regarding laser-cooling of molecules. Di Rosa et al. [5] have made a study concentrating on CaH, where they present recent high-resolution, molecular-beam-based measurements of low-J rotational lines within the A2P–X2R (0,0) band of CaH. They reported hyperfine separations in the A-state, as important to laser-cooling spectroscopy, and centroidal transition frequencies for comparison with existing values. They outlined a possible magneto-optical trap for CaH. Friedrich et al. [6] performed Zeeman spectroscopy of CaH molecules in a magnetic trap and they have
* Corresponding author. Fax: +46 8 5537 8216. E-mail address: [email protected] (L.-E. Berg). 0022-2852/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2009.06.004
also reported an experiment in which 108 CaH molecules have been confined in a magnetic trap at a temperature of 400 mK [7]. The A2P–X2R and B2R–X2R band systems of CaH were reinvestigated in 1974 [8,9]. In 1976 some new bands of the B2R–X2R transition were recorded by means of laser excitation spectroscopy [10]. So far, the only radiative lifetime measurements of the lowest states of the alkaline earth hydrides reported are on the B2R states of CaH [11,12], SrH [13,14] and BaH [15]. However, a great number of time-resolved studies have been performed on the low-lying states of the alkaline earth monohalides [16–22]. In the present investigation, we have measured the radiative lifetimes of the first vibrational levels of the A2P state of CaH by using an opto-acoustically pulsed cw dye laser and by applying the delayed coincidence technique. The spectra of the A2P–X2R and B2R–X2R transitions of CaH appear in the red and orange wavelength regions (580–720 nm) with the dominating band heads belonging to the A2P–X2R (0,0) band at 691.0 and 695.0 nm and the B2R–X2R (0,0) band at 635.0 nm. The following zero-pressure lifetimes were determined for the A2P state: st=0 = 33.2 (±3.2) ns and st=01 = 37.7 (±5.2) ns with the corresponding oscillator strengths, f(0,0) = 0.43 (±0.04) and f(1,1) = 0.44 (±0.07). A small resistance furnace was used to produce the calcium hydride molecules. The furnace was heated to a temperature of just above 1000 K. Metallic calcium was placed in the furnace and heated in an atmosphere of pure hydrogen. The maximum magnetic field in the center of the reactor was just of the order of 150 lT, so spectroscopic effects due to the weak magnetic field could be neglected. The furnace pressure was varied between 0.1 mbar and 15 mbar and was carefully controlled by pressure devices such as a Pirani gauge, a mercury manometer and a pressure
106
M. Liu et al. / Journal of Molecular Spectroscopy 257 (2009) 105–107
transducer. We estimate the pressure measurements to have an accuracy of <7%. At ‘‘higher” pressures around 1.0 mbar the uncertainties are (±0.08 mbar. A cw dye laser was used to perform excitations in the 630–650 nm wavelength region to excite the A2P–X2R (0,0) and B2R–X2R (0,0) transitions. The A2P1/2 and A2P3/2 states are closely located to each other with DTe = 80 cm1. By using the results from Berg and Klynning [2,3], unblended spectral lines of the P(0,0) branches were chosen lying close to the band heads at 702.9 nm and 703.4 nm of the P1(0,0) and P2(0,0) branches. The corresponding fluorescence was then observed in the R1(0,0) branches. A large number of pairs of P–R lines were chosen and the lifetimes were determined. The B system has been examined earlier and measurements were then performed close to the P1(0,0) band head at the wavelength 638.9 nm and to the P2(0,0) band head at 638.2 nm, when examining the t0 = 0 level of the B2R–X2R transition. Excitations of unblended spectral lines belonging to the (1,1) A2P–X2R transition were also examined. Several pairs were examined, but no differences in lifetimes were observed within the error limits. A computer-operated Czerny–Turner monochromator (0.5 m, F = 10), supplied with a holographic grating (dl = 3000 lines mm1 and R = (k/Dk) = 5 104 to the first order), was used in the experiments. The slit width of the monochromator was held rather broad, around 1 mm, in order to eliminate the flight-out-of-view effect [22,23]. A cooled Hamamatsu R943 photomultiplier tube was used. The excitations were performed using an Ar+ laser pumped Coherent Radiation CR 599 dye laser, supplied with a thin standard etalon. The dyes used were Oxazine 170 and Oxazine 4. When wavelengths above 700 nm were applied, a tunable Ti:Sapphire laser (Spectra Physics 3900S) was used. The laser power of the dye laser was around 250 mW, when running in cw operation. In each experiment precautions were taken in order to eliminate laser stray light. In all situations, the P–R separations were checked against known molecular constants [9] and published wavelength tables [8]. An opto-acoustic modulator (NEWPORT EOS) was used, having a falling time of the pulses <4 ns in order to create the 50 ns wide pulses. The system repetition frequency was set to l–5 MHz. The signal was obtained using a time-to-amplitude-converter (TAC, Tennelec 527) and stored in a PC equipped with an Ortec
Fig. 1. Fluorescence decay curve of the CaH A2P–X2R transition at a pressure of 0.145 mbar. The solid line corresponds to a fit consisting of a constant background, and an exponential decay. The dashed line represents the constant background and the decay curve.
Table 1 Experimentally derived radiative lifetimes (ns) at zero-pressure for CaH in the A2P state and oscillator strengths f(t,t”), and transition moments of the A2P ?X2R transitions of CaH. Molecule Lifetime (ns)
CaH
f (0,0) f (1,1) f(2,2) R2e (rtt”) (au)2a
st’=0 st=1
CaH
A2P½ A2P3/2
B2R
33.2 (±3.2) 33.7 (±5.2) 0.43 (±0.04) 0.44 (±0.07)
58.1 (±1.9) [12] 58.4 (±2.2) [12] 0.105 (±0.004) 0.105 (±0.004) 0.105 (±0.004) 2.2
4.95
The uncertainties, representing one standard deviation, are given in parenthesis. Transition moments have been calculated using the m00 values. a 1 (au)2 6.46 (debye)2 7.19 1059 C2m2.
EG&G MCA system. The time resolution was varied between 0.500 and 4.10 ns per channel in different runs. For each excitation wavelength a series of experiments were performed at different pressures, where the sampling time was varied between 10 and 25 min. For more details see references [12–15,17–19]. Both deconvolution and non-deconvolution was applied in calculating the lifetimes of the different states. However, the difference in the lifetimes determined with the two methods was less than 0.3 ns. In Fig. 1 a typical fluorescence decay curve of the A2P– X2R transition obtained at 0.145 mbar is given and the derived lifetime was 31.90 (±0.32) ns. The radiative lifetime s of an excited vibrational level, t, for a diatomic molecule is related to the transition probabilities (Att”) by the following expression:
1
st
¼
X t
Att
X t
m3tt qtt R2e ðrtt Þ
64p4 g 0 3h g 00
ð1Þ
The Franck–Condon (FC) factors (qtt”), the r-centroids (rtt”) and the oscillator strengths (ftt”) of the A2P–X2R transitions were calculated by using the results from our measurement of radiative lifetimes together with published molecular constants [9]. The statistical factor, g´/g00 is equal to 1 for all transitions except for a P–R transition where it is equal to 2. The FC factor of the (0,0) transition is 0.98 and for the (0,1) transition 0.02. The derived CaH A2P– X2R oscillator strengths are given in Table 1, where also the corresponding parameters of the B2P–X2R transition are given (see Rice [26]). A Stern–Vollmer plot of the inverted decay rate versus pressure resulted in a pressure-induced depopulation rate of 1.3 (±0.1) 109 cm3 molecule1 s1 (8.7 (±1.3) MHz/mbar), in good agreement with the result of the B2R state measurements and of Klynning et al. [11] respectively Weijnitz (33.6 (±8.7) ns, 9.2 (±2.2) MHz/mbar) [24,25]. The lifetime measurements of the A2P12 and A2P32 states (Table 1) differ less than one standard deviation of fit, why we give the same lifetime of both states, st=0 = 33.2 (±3.2) ns in the Table. The t0 = 0, 1 and 2 levels B2R state of CaH have been measured earlier [12], with the following results: B2R state; st=0 = 58.1 (±1.9) ns, st=1 = 58.4 (±2.2) ns, st=2 = 58.5 (±2.6) ns. In this experiment, the standard error of fit in a normal single run with sufficient data points is less than 0.8 ns. The radiative lifetimes of the A 2 P and B2R states are given in Table 1. The vibrational constants of the A2P, B2R and X2R states of CaH are 1333, 1285 and 1298 cm1 [9], respectively, so the radiative rate constant is supposed to be almost constant when increasing the vibrational quantum number, t0 according to Fleming [27]. The result for the A2P state is: A00 = 30.1 (±4.9) ls2, A11 = 26.5(±3.7) ls2. The results regarding the B2R state is: A00 = 17.2 (±0.6) ls2, A11 = 17.1(±0.7) ls2, A22= 17.1(±0.8) ls2. So, by accurately measuring the radiative lifetimes of laser-excited diatomic halides and hydrides, we have obtained information regarding molecular opacity for astronomical purposes that for long has been one of the wishes from astronomers working with stellar atmospheres.
M. Liu et al. / Journal of Molecular Spectroscopy 257 (2009) 105–107
Acknowledgments We thank Peter Weijnitz and Mats Doverstål for discussions about their work on the CaH molecule. References [1] P. Sotirowski, Astron. Astrophys. Suppl. Ser. 6 (1972) 85. [2] A.J. Burgasser, J.D. Kirkpatrick, J. Liebert, A. Burrows, Astrophys. J. 594 (2003) 510. [3] V.M. Woolf, G. Wallerstein, Month. Not. R. Astron. Soc. 350 (2004) 1365. [4] R. Pereira, S. Skowronek, A. GonzalezUrena, A. Pardo, J.M.L. Poyato, A.H. Pardo, J. Mol. Spec. 212 (2002) 17. [5] M.D. Di Rosa, E. Phys. J. 31 (2004) 395. [6] B. Friedrich, J.D. Weinstein, R. DeCarvalho, J.M. Doyle, J. Chem. Phys. 110 (1999) 2376. [7] R. DeCarvalho, J.D. Weinstein, B. Friedrich, J.M. Doyle, Proc. SPIE 4634 (2002) 46. [8] L.-E. Berg, L. Klynning, Astrophys. Suppl. Ser. 13 (1974) 325.
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
107
L.-E. Berg, L. Klynning, Phys. Scripta 10 (1974) 331. L.-E. Berg, H. Martin, L. Klynning, Opt. Commun. 17 (1976) 320. L. Klynning, H. Martin, P. Nyle’n, P. Royen, Phys. Scripta 25 (1982) 362. L.-E. Berg, K. Ekvall, A.S. Kelly, Chem. Phys. Lett. 257 (1996) 351. L.-E. Berg, K. Ekvall, A. Hishikawa, S. Kelly, Chem. Phys. Lett. 255 (1996) 419. T. Pauchard, M. Liu, O. Launila, L.-E. Berg, J. Mol. Spec. 247 (2008) 181–183. L.-E. Berg, K. Ekvall, A. Hishikawa, S. Kelly, Phys. Scripta 55 (1997) 269. P.J. Dagdigian, H.W. Cruse, R.N. Zare, J. Chem. Phys. 60 (1974) 2330. L.-E. Berg, H. Martin, P. Royen, P. Weijnitz, Chem. Phys. Lett. 132 (1986) 261. L.-E. Berg, P. Royen, P. Weijnitz, Chem. Phys. Lett. 159 (1989) 175. L.-E. Berg, P. Royen, P. Weijnitz, Mol. Phys. 69 (1990) 385. G. Gustavsson, H. Martin, P. Weijnitz, Opt. Commun. 67 (1988) 112. L.-E. Berg, J.-C. Chanteloup, A. Hishikawa, T. Olsson, P. Royen, Mol. Phys. 79 (1993) 721. L.-E. Berg, K. Ekvall, T. Hansson, A. Iwamae, V. Zengin, D. Husain, P. Royen, Chem. Phys. Lett. 248 (1993) 283. B. Meng, J. Quant. Spectrosc. Radiat. Transfer 41 (1989) 303. P. Royen, P. Weijnitz, Phys. Scripta 46 (1992) 399. Private communication. Peter Weinitz , M. Doverstål. Stockholm University. S.F. Rice, H. Martin, R.W. Field, J. Chem. Phys. 82 (1985) 5023. G.R. Fleming, O.L.J. Gijzeman, S.H. Lin, Chem. Phys. Lett. 21 (1973) 527.
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Note
Time-resolved study of the A2P state of CaH by laser spectroscopy M. Liu a, T. Pauchard a, M. Sjödin a, O. Launila a, P. van der Meulen b, L.-E. Berg a,* a b
Department of Applied Physics, KTH (Royal Institute of Technology), Roslagstullsbacken 21, S-106 91 Stockholm, Sweden Department of Physics, Stockholm University, S-106 91 Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 12 May 2009 In revised form 3 June 2009 Available online 13 June 2009 Keywords: Spectroscopy Life time Radicals Solar spectra
a b s t r a c t A time-resolved experiment on the A2P state of gaseous calcium hydride has been performed by applying laser spectroscopic methods. The following zero-pressure lifetime was obtained for the CaH A2P state: st=0 = 33.2 (±3.2) ns and st=1 = 33.7 (±5.2) ns. The lifetime was found to be the same for the A2P½ and A2P3/2 states. Ó 2009 Elsevier Inc. All rights reserved.
Gaseous CaH has been the subject of many experimental studies in the past. It is a molecule of great interest to chemists, physicists and of course to astrophysicists, due to its abundance in stellar atmospheres. Experiments to obtain electronic, vibrational and rotational constants as well as to perform lifetime measurements are thus of major importance. Bands of CaH and CaD have been observed in the solar spectrum and also in certain types of stars, where high abundances of these molecules have been registered [1]. Burgasser et al. have presented spectra for T dwarfs observed using the Keck I 10 m Telescope, where they observed a variety of features related to bands of CaH [2]. The chemical abundance of CaH in Kapteyn’s Star [3] has been studied by Woolf and Wallerstein. By applying a supersonic molecular beam, rotationally resolved spectra of the CaH radical have been reported. High-energy resolution of the (1 + 1’) REMPI spectra corresponding to the A2P–X2R transitions were measured, which allowed for the first time a clear and precise analysis of the low-rotational part [4]. The A2P–X2R (0,0) band of CaH has rather recently been shown to be a candidate regarding laser-cooling of molecules. Di Rosa et al. [5] have made a study concentrating on CaH, where they present recent high-resolution, molecular-beam-based measurements of low-J rotational lines within the A2P–X2R (0,0) band of CaH. They reported hyperfine separations in the A-state, as important to laser-cooling spectroscopy, and centroidal transition frequencies for comparison with existing values. They outlined a possible magneto-optical trap for CaH. Friedrich et al. [6] performed Zeeman spectroscopy of CaH molecules in a magnetic trap and they have
* Corresponding author. Fax: +46 8 5537 8216. E-mail address: [email protected] (L.-E. Berg). 0022-2852/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2009.06.004
also reported an experiment in which 108 CaH molecules have been confined in a magnetic trap at a temperature of 400 mK [7]. The A2P–X2R and B2R–X2R band systems of CaH were reinvestigated in 1974 [8,9]. In 1976 some new bands of the B2R–X2R transition were recorded by means of laser excitation spectroscopy [10]. So far, the only radiative lifetime measurements of the lowest states of the alkaline earth hydrides reported are on the B2R states of CaH [11,12], SrH [13,14] and BaH [15]. However, a great number of time-resolved studies have been performed on the low-lying states of the alkaline earth monohalides [16–22]. In the present investigation, we have measured the radiative lifetimes of the first vibrational levels of the A2P state of CaH by using an opto-acoustically pulsed cw dye laser and by applying the delayed coincidence technique. The spectra of the A2P–X2R and B2R–X2R transitions of CaH appear in the red and orange wavelength regions (580–720 nm) with the dominating band heads belonging to the A2P–X2R (0,0) band at 691.0 and 695.0 nm and the B2R–X2R (0,0) band at 635.0 nm. The following zero-pressure lifetimes were determined for the A2P state: st=0 = 33.2 (±3.2) ns and st=01 = 37.7 (±5.2) ns with the corresponding oscillator strengths, f(0,0) = 0.43 (±0.04) and f(1,1) = 0.44 (±0.07). A small resistance furnace was used to produce the calcium hydride molecules. The furnace was heated to a temperature of just above 1000 K. Metallic calcium was placed in the furnace and heated in an atmosphere of pure hydrogen. The maximum magnetic field in the center of the reactor was just of the order of 150 lT, so spectroscopic effects due to the weak magnetic field could be neglected. The furnace pressure was varied between 0.1 mbar and 15 mbar and was carefully controlled by pressure devices such as a Pirani gauge, a mercury manometer and a pressure
106
M. Liu et al. / Journal of Molecular Spectroscopy 257 (2009) 105–107
transducer. We estimate the pressure measurements to have an accuracy of <7%. At ‘‘higher” pressures around 1.0 mbar the uncertainties are (±0.08 mbar. A cw dye laser was used to perform excitations in the 630–650 nm wavelength region to excite the A2P–X2R (0,0) and B2R–X2R (0,0) transitions. The A2P1/2 and A2P3/2 states are closely located to each other with DTe = 80 cm1. By using the results from Berg and Klynning [2,3], unblended spectral lines of the P(0,0) branches were chosen lying close to the band heads at 702.9 nm and 703.4 nm of the P1(0,0) and P2(0,0) branches. The corresponding fluorescence was then observed in the R1(0,0) branches. A large number of pairs of P–R lines were chosen and the lifetimes were determined. The B system has been examined earlier and measurements were then performed close to the P1(0,0) band head at the wavelength 638.9 nm and to the P2(0,0) band head at 638.2 nm, when examining the t0 = 0 level of the B2R–X2R transition. Excitations of unblended spectral lines belonging to the (1,1) A2P–X2R transition were also examined. Several pairs were examined, but no differences in lifetimes were observed within the error limits. A computer-operated Czerny–Turner monochromator (0.5 m, F = 10), supplied with a holographic grating (dl = 3000 lines mm1 and R = (k/Dk) = 5 104 to the first order), was used in the experiments. The slit width of the monochromator was held rather broad, around 1 mm, in order to eliminate the flight-out-of-view effect [22,23]. A cooled Hamamatsu R943 photomultiplier tube was used. The excitations were performed using an Ar+ laser pumped Coherent Radiation CR 599 dye laser, supplied with a thin standard etalon. The dyes used were Oxazine 170 and Oxazine 4. When wavelengths above 700 nm were applied, a tunable Ti:Sapphire laser (Spectra Physics 3900S) was used. The laser power of the dye laser was around 250 mW, when running in cw operation. In each experiment precautions were taken in order to eliminate laser stray light. In all situations, the P–R separations were checked against known molecular constants [9] and published wavelength tables [8]. An opto-acoustic modulator (NEWPORT EOS) was used, having a falling time of the pulses <4 ns in order to create the 50 ns wide pulses. The system repetition frequency was set to l–5 MHz. The signal was obtained using a time-to-amplitude-converter (TAC, Tennelec 527) and stored in a PC equipped with an Ortec
Fig. 1. Fluorescence decay curve of the CaH A2P–X2R transition at a pressure of 0.145 mbar. The solid line corresponds to a fit consisting of a constant background, and an exponential decay. The dashed line represents the constant background and the decay curve.
Table 1 Experimentally derived radiative lifetimes (ns) at zero-pressure for CaH in the A2P state and oscillator strengths f(t,t”), and transition moments of the A2P ?X2R transitions of CaH. Molecule Lifetime (ns)
CaH
f (0,0) f (1,1) f(2,2) R2e (rtt”) (au)2a
st’=0 st=1
CaH
A2P½ A2P3/2
B2R
33.2 (±3.2) 33.7 (±5.2) 0.43 (±0.04) 0.44 (±0.07)
58.1 (±1.9) [12] 58.4 (±2.2) [12] 0.105 (±0.004) 0.105 (±0.004) 0.105 (±0.004) 2.2
4.95
The uncertainties, representing one standard deviation, are given in parenthesis. Transition moments have been calculated using the m00 values. a 1 (au)2 6.46 (debye)2 7.19 1059 C2m2.
EG&G MCA system. The time resolution was varied between 0.500 and 4.10 ns per channel in different runs. For each excitation wavelength a series of experiments were performed at different pressures, where the sampling time was varied between 10 and 25 min. For more details see references [12–15,17–19]. Both deconvolution and non-deconvolution was applied in calculating the lifetimes of the different states. However, the difference in the lifetimes determined with the two methods was less than 0.3 ns. In Fig. 1 a typical fluorescence decay curve of the A2P– X2R transition obtained at 0.145 mbar is given and the derived lifetime was 31.90 (±0.32) ns. The radiative lifetime s of an excited vibrational level, t, for a diatomic molecule is related to the transition probabilities (Att”) by the following expression:
1
st
¼
X t
Att
X t
m3tt qtt R2e ðrtt Þ
64p4 g 0 3h g 00
ð1Þ
The Franck–Condon (FC) factors (qtt”), the r-centroids (rtt”) and the oscillator strengths (ftt”) of the A2P–X2R transitions were calculated by using the results from our measurement of radiative lifetimes together with published molecular constants [9]. The statistical factor, g´/g00 is equal to 1 for all transitions except for a P–R transition where it is equal to 2. The FC factor of the (0,0) transition is 0.98 and for the (0,1) transition 0.02. The derived CaH A2P– X2R oscillator strengths are given in Table 1, where also the corresponding parameters of the B2P–X2R transition are given (see Rice [26]). A Stern–Vollmer plot of the inverted decay rate versus pressure resulted in a pressure-induced depopulation rate of 1.3 (±0.1) 109 cm3 molecule1 s1 (8.7 (±1.3) MHz/mbar), in good agreement with the result of the B2R state measurements and of Klynning et al. [11] respectively Weijnitz (33.6 (±8.7) ns, 9.2 (±2.2) MHz/mbar) [24,25]. The lifetime measurements of the A2P12 and A2P32 states (Table 1) differ less than one standard deviation of fit, why we give the same lifetime of both states, st=0 = 33.2 (±3.2) ns in the Table. The t0 = 0, 1 and 2 levels B2R state of CaH have been measured earlier [12], with the following results: B2R state; st=0 = 58.1 (±1.9) ns, st=1 = 58.4 (±2.2) ns, st=2 = 58.5 (±2.6) ns. In this experiment, the standard error of fit in a normal single run with sufficient data points is less than 0.8 ns. The radiative lifetimes of the A 2 P and B2R states are given in Table 1. The vibrational constants of the A2P, B2R and X2R states of CaH are 1333, 1285 and 1298 cm1 [9], respectively, so the radiative rate constant is supposed to be almost constant when increasing the vibrational quantum number, t0 according to Fleming [27]. The result for the A2P state is: A00 = 30.1 (±4.9) ls2, A11 = 26.5(±3.7) ls2. The results regarding the B2R state is: A00 = 17.2 (±0.6) ls2, A11 = 17.1(±0.7) ls2, A22= 17.1(±0.8) ls2. So, by accurately measuring the radiative lifetimes of laser-excited diatomic halides and hydrides, we have obtained information regarding molecular opacity for astronomical purposes that for long has been one of the wishes from astronomers working with stellar atmospheres.
M. Liu et al. / Journal of Molecular Spectroscopy 257 (2009) 105–107
Acknowledgments We thank Peter Weijnitz and Mats Doverstål for discussions about their work on the CaH molecule. References [1] P. Sotirowski, Astron. Astrophys. Suppl. Ser. 6 (1972) 85. [2] A.J. Burgasser, J.D. Kirkpatrick, J. Liebert, A. Burrows, Astrophys. J. 594 (2003) 510. [3] V.M. Woolf, G. Wallerstein, Month. Not. R. Astron. Soc. 350 (2004) 1365. [4] R. Pereira, S. Skowronek, A. GonzalezUrena, A. Pardo, J.M.L. Poyato, A.H. Pardo, J. Mol. Spec. 212 (2002) 17. [5] M.D. Di Rosa, E. Phys. J. 31 (2004) 395. [6] B. Friedrich, J.D. Weinstein, R. DeCarvalho, J.M. Doyle, J. Chem. Phys. 110 (1999) 2376. [7] R. DeCarvalho, J.D. Weinstein, B. Friedrich, J.M. Doyle, Proc. SPIE 4634 (2002) 46. [8] L.-E. Berg, L. Klynning, Astrophys. Suppl. Ser. 13 (1974) 325.
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
107
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