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Millimeter wave spectra of MgS and CaS
CHEMICAL PHYSICS LETTERS
Volume 159, number 5,6
MILLIMETER Shuro
TAKANO,
WAVE SPECTRA
21 July 1989
OF MgS AND CaS
Satoshi YAMAMOTO
and Shuji SAITO
Department oJAstrophy.sics, Faculty of Science, Nagoya University, Chikusa, Nagoya 464-01, Japan Received 31 March 1989; in final form 28 April 1989
The pure rotational spectra of MgS and CaS (both in the X ‘L+ state) were observed in the frequency region of 200 to 300 GHz by using a source-modulated microwave spectrometer and a high-temperature cell. MgS was produced by the reaction between magnesium vapor and sulfur vapor, while CaS was produced by the reaction between calcium vapor and OCS. Seven rotational transitions were observed for MgS in the ground vibrational state, and 8 to 10 transitions for CaS in the vibrational states of v= 0 to 3. Least-squares analyses of the observed spectral lines yielded the rotational constants and the centrifugal distortion constants for both molecules.
Recently four metal halides, NaCl, AlCl, AlF and KCl, have been detected toward the red giant star IRC+ 10216 [ 11. This suggests the possibility that molecules bearing low-cosmic-abundance atoms may be found in circumstellar clouds or molecular clouds. Since the cosmic abundances of Mg and Ca are comparable with those of Si and Al, respectively, molecules containing Mg and Ca might be detectable in interstellar space. MgO [ 2,3 ] and CaO [ 4 ] have been searched for toward stars and molecular clouds, but neither of them have been detected. According to the chemical equilibrium model proposed by Tsuji [ 51, CaS has a higher abundance than that of CaO under certain conditions in stellar atmospheres. Since the molecular constants of CaS [ 61 and MgS [ 7 ] have been known only from high-resolution optical spectroscopy, we studied the pure rotational spectra of these molecules by microwave spectroscopy, aiming at determination of their accurate transition frequencies, which may be useful for searches of interstellar MgS and CaS. In this Letter we report the laboratory observation of the millimeter wave spectra of MgS and CaS. Details of the spectrometer used were given in a previous paper [ 8 1. The absorption cell is a cylindrical vacuum chamber of 40 cm inner diameter and 30 cm height, equipped with an oven bearing a crucible to produce the metal vapor (fig. 1). The crucible is placed in the center of the chamber, just be0 009-2614/89/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
A
I
-LJ -_ 5 -
--__-
1Cm
Fig. 1. Schematic drawing of the oven. (A) Glass tube containing sulfur for h4& (B) crucible, (C) ring-shaped Cu electrodes, (D ) Ta foil (heater), (E) Ta radiation shield, (F) ceramic spacers.
low the path of microwave transmission, and is surrounded by a tantalum foil 0.05 mm thick which serves as a resistance heater. The chamber has eight ports separated from one another by 45”. A pair of facing ports, which are sealed with Teflon lenses, serves as input and output windows for microwave transmission. Other ports’are used for introduction of reactant or buffer gases, pumping of reaction products, input of electricity for the heater, and for a Pirani gauge. MgS was produced by the reaction of magnesium vapor with sulfur vapor. The magnesium vapor was, produced by heating a magnesium metal ribbon in the carbon crucible to its melting point, and the sulfur vapor was produced by heating sulfur powder in a glass tube (fig_ 1) . No line was observed when OCS B.V.
563
Volume 159, number 5,6
CHEMICAL PHYSICS LETTEXS
alone was added to magnesium vapor. The spectral line of MgS became stronger when the buffer gas Ar was introduced in the chamber. The optimum pressure of the gas was about 10 mTorr (1.3 Pa). This fact indicates that the third body is effective for the production of MgS or for the retardation of the reactivity of MgS. CaS was produced by reaction of calcium vapor with OCS of about 10 mTorr ( 1.3 Pa). White chemiluminescence was clearly recognized in this reaction. A stainless steel crucible was used instead of the carbon crucible to produce calcium vapor by heating to its melting point, since calcium reacts with solid carbon. The lines were also observed when CS2 or H# was introduced instead of OCS, but the line intensity was much weaker. The spectral line intensity of CaS was so strong that the rotational lines were observed for four vibrational states, from v=O to 3. A typical signal to noise ratio was about 100 for the transition of J=26-25 in the ground vibrational state at 275 GHz. The spectral line intensity becomes weaker by about one half or one third as the vibrational quantum number increases by one. The intensity behavior with the vibrational quantum number suggests that the effective vibrational temperature of CaS is 600-900 K. Seven rotational transitions of MgS in the ground vibrational state were observed in the frequency region of 224-368 GHz, as listed in table 1. An example of the spectral lines observed is shown in fig. 2. The rotational constant and the centrifugal dis-
Table 1 Observed transition frequencies of MgS (D= 0) a) Transition J
v&a
Au b’
14-13 16-15 17-16 18-17 19-18 22-21 23-22
224103.178(45) c, 256086.116( 19) 272072.920(29) 288056.376(26) 304036.237( 29) 351952.415(30) 367915.973(43)
0.019 - 0.006 -0.017 0.000 -0.003 0.015 - 0.007
a) In MHz. b’ Av= vh - vcailc. ” The values in parentheses are one standard deviation of the frequency measurements and apply to the last digits of the frequencies.
564
I
I 224101
21 July 1989 I
I
224103
I
I
224105
4
FREQUENCYlMHz)
Pig. 2. The 3= 14-l 3 rotational spectral line of MgS. The integration time is about 25 s.
tortion constant were determined from the observed spectral lines by a least-squares method. The standard deviation of the fit was 0.014 MHz. The molecular constants determined are given in table 2. The molecular constants agree with those obtained from the electronic absorption spectrum [ 7 1, but the accuracy of the constants has been improved by two orders of magnitude. Eight to ten rotational transitions of CaS in the region of 198-306 GHz were observed for each vibrational state, as listed in table 3. The rotational constants and the centrifugal distortion constants determined by least-squares fits are shown in table 2. The standard deviations of the fits were 0,017, 0.025, 0.018 and 0.030 MHz for the v=O, 1, 2 and 3 states, respectively. They also agree well with those obtained from the electronic absorption spectrum
[61. The rotational constant B, is expressed in the power series expansion of V+ l/2. A least-squares fit of the rotational constants for CaS gave the equilibrium rotational constant and the vibration-rotation constants to be B,=5296.6032( 15), a,=24.7921(22), and y,=-0.06959(60) MHz, where the values in parentheses are three times the standard deviations of the fit. The equilibrium internuclear distance of CaS was derived to be 2.317751( 1) A. The error given in parentheses originates mainly from the er-
CHEMICAL
Volume 159, number $6 Table 2 Molecular
constants
of M@ and CaS a) MW b’
II
Optical
MBS
0
& &
8006.9278(15) 0.00X2744( 19)
CaS
0
Es
5284.1898(15) 0.0031021(11)
DO Ho 81 DI HI
2
& DZ Hz
5234.1881(18) 0.0031341(14)
Bs D3
5208.9781(29) 0.0031496(22)
8007.25(39) ‘) 0.008280 (30) ‘) 5283.87( 18) d, 0.003058( 12) d, -1.01(20)x10-~d’
-
1
3
21 July 1989
PHYSICS LETTERS
5258.66(24) d’ 0.0030609(90) d, -1.271(75)x10-96’
5259.2581(21) 0.0031182(16) -
5233.69(33) ‘) 0.003067 (30) ‘) -1.66(75)x 1O-9d’
-
-
*) In MHz. The values in parentheses are three standard deviations and apply to the last digits of the constants. b, This study. c) Ref. [ 71. The values, originally reported in cm-‘, are converted to MHz. d, Ref. [6]. The values, originally reported in cm-‘, are converted to MHz.
rors in the equilibrium rotational constant and the fundamental physical constants. The small contribution of electrons to the moment of inertia was not included in the calculation. The four centrifugal distortion constants were also fitted to the power series expansion of v+ l/2 as D,(MHz)=0.00309391(18)
~lnol.r.mb~\o-d-+oN400N
~ooooooooc 5ddddddddl
+0.00001644(26)(u+1/2) -0.000000149(71)(u+
I
l/2)2.
I
I
I
I
(1)
The first term is De, and the coeffkient of the second term A, The vibrational frequency W, and the anharmonicity constant WJ~ were calculated to be 462.3 and 1.634 cm- ’ , respectively, from B,, De and (Y, [ 9 1. They are consistent with those obtained from the electronic absorption spectrum [ 6 1. MgS could not be observed by the reaction of magnesium vapor with OCS. The heat of reaction of Mg+ OCS+MgS + CO in the gas phase is estimated to be endothermic by more than 80 kJ/mol. On the other hand, the reaction of magnesium vapor with S2 is exothermic. The heat of the three-body reaction 565
Volume 159, number
5,6
CHEMICAL
PHYSICS LIZ-l-I-ERS
2Mg+S,-+ZMgS in the gas phase is estimated to be exothermic by less than 38 kJ/mol. For CaS, which was readily produced by the reaction of calcium vapor with OCS, the heat of reaction is also estimated to be exothermic by about 23 kJ/mol. Therefore, these exothermic reactions are considered to be effective for the production of MgS and CaS. The molecular constants obtained in the present study enabled us to calculate the frequencies of lowJ transitions with an accuracy sufficient for the radioastronomical search for MgS and CaS. T’he spectral data toward relatively warm sources, our galactic center SgrB2 (0.080 K), the red giant star IRC + 102 16 (0.068 K), and the star-forming region W51A (0.052 K) [lo] for MgS (J=7-6), and IRC + 102 16 [ 111 for CaS (J= 7-6 ) were examined at the relevant frequencies calculated. The values in parentheses are root mean squares of the noise in antenna temperature. No lines corresponding to the transitions of MgS or CaS were found above the signal to noise ratio levels attained at present. We are grateful to N. Kaifu for the spectral line data obtained by the 45 m radiotelescope at Nobeyama Radio Observatory. We also thank the staff
566
21 July 1989
members of the Instrumental Development Center of Faculty of Science, Nagoya University for their construction of the cell used in this study. This study was supported by a Grant-in-Aid from the Ministry of Education, Science, and Culture (Nos. 62470015, 62740277,63632514, and 63540192).
References [ I] J. Cemicharo and M. GuClin, Astron. Astrophys. 183 ( 1987) LIO. [2] BE Tumer and T.C. Steimle, Astrophys. J. 299 (1985) 956. [3] T.J. M&r, J. EUd&, A. Hjahwson and H. Olofbson, Astron. Astrophys. 182 (1987) 143. [4] W.H. Hocking, G. Winnewisser, E. Churchwell and J. Percival, Astron. Astrophys. 75 (1979) 268. [ 5] T. Tsuji, Astron. Astrophys. 23 (1973) 411. [ 6 1 R.C. Blues and RF. E%arrow,Trans. Faraday Sot. 65 ( 1969 )
[ 71 ~?Marcanc~
and R.F. Barrow, Trans. Faraday Sot. 66 (1970) 2936. [S] S. Yamamoto and S. Saito, J. Chem. Phys. 89 (1988) 1936. [9] W. Gordy and R.L. Cook, Microwave molecular spectra, 3rd Ed. ( Wiley-Interscience, New York, 1984) ch. 4. [ lo] N. Kaifu, private communication. [ 111J. Cemicharo, C. Kahane, J. Gbmez-Gonzalez and M. Gu6li11, Astron. Astrophys. 164 (1986) Ll.
Volume 159, number 5,6
MILLIMETER Shuro
TAKANO,
WAVE SPECTRA
21 July 1989
OF MgS AND CaS
Satoshi YAMAMOTO
and Shuji SAITO
Department oJAstrophy.sics, Faculty of Science, Nagoya University, Chikusa, Nagoya 464-01, Japan Received 31 March 1989; in final form 28 April 1989
The pure rotational spectra of MgS and CaS (both in the X ‘L+ state) were observed in the frequency region of 200 to 300 GHz by using a source-modulated microwave spectrometer and a high-temperature cell. MgS was produced by the reaction between magnesium vapor and sulfur vapor, while CaS was produced by the reaction between calcium vapor and OCS. Seven rotational transitions were observed for MgS in the ground vibrational state, and 8 to 10 transitions for CaS in the vibrational states of v= 0 to 3. Least-squares analyses of the observed spectral lines yielded the rotational constants and the centrifugal distortion constants for both molecules.
Recently four metal halides, NaCl, AlCl, AlF and KCl, have been detected toward the red giant star IRC+ 10216 [ 11. This suggests the possibility that molecules bearing low-cosmic-abundance atoms may be found in circumstellar clouds or molecular clouds. Since the cosmic abundances of Mg and Ca are comparable with those of Si and Al, respectively, molecules containing Mg and Ca might be detectable in interstellar space. MgO [ 2,3 ] and CaO [ 4 ] have been searched for toward stars and molecular clouds, but neither of them have been detected. According to the chemical equilibrium model proposed by Tsuji [ 51, CaS has a higher abundance than that of CaO under certain conditions in stellar atmospheres. Since the molecular constants of CaS [ 61 and MgS [ 7 ] have been known only from high-resolution optical spectroscopy, we studied the pure rotational spectra of these molecules by microwave spectroscopy, aiming at determination of their accurate transition frequencies, which may be useful for searches of interstellar MgS and CaS. In this Letter we report the laboratory observation of the millimeter wave spectra of MgS and CaS. Details of the spectrometer used were given in a previous paper [ 8 1. The absorption cell is a cylindrical vacuum chamber of 40 cm inner diameter and 30 cm height, equipped with an oven bearing a crucible to produce the metal vapor (fig. 1). The crucible is placed in the center of the chamber, just be0 009-2614/89/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
A
I
-LJ -_ 5 -
--__-
1Cm
Fig. 1. Schematic drawing of the oven. (A) Glass tube containing sulfur for h4& (B) crucible, (C) ring-shaped Cu electrodes, (D ) Ta foil (heater), (E) Ta radiation shield, (F) ceramic spacers.
low the path of microwave transmission, and is surrounded by a tantalum foil 0.05 mm thick which serves as a resistance heater. The chamber has eight ports separated from one another by 45”. A pair of facing ports, which are sealed with Teflon lenses, serves as input and output windows for microwave transmission. Other ports’are used for introduction of reactant or buffer gases, pumping of reaction products, input of electricity for the heater, and for a Pirani gauge. MgS was produced by the reaction of magnesium vapor with sulfur vapor. The magnesium vapor was, produced by heating a magnesium metal ribbon in the carbon crucible to its melting point, and the sulfur vapor was produced by heating sulfur powder in a glass tube (fig_ 1) . No line was observed when OCS B.V.
563
Volume 159, number 5,6
CHEMICAL PHYSICS LETTEXS
alone was added to magnesium vapor. The spectral line of MgS became stronger when the buffer gas Ar was introduced in the chamber. The optimum pressure of the gas was about 10 mTorr (1.3 Pa). This fact indicates that the third body is effective for the production of MgS or for the retardation of the reactivity of MgS. CaS was produced by reaction of calcium vapor with OCS of about 10 mTorr ( 1.3 Pa). White chemiluminescence was clearly recognized in this reaction. A stainless steel crucible was used instead of the carbon crucible to produce calcium vapor by heating to its melting point, since calcium reacts with solid carbon. The lines were also observed when CS2 or H# was introduced instead of OCS, but the line intensity was much weaker. The spectral line intensity of CaS was so strong that the rotational lines were observed for four vibrational states, from v=O to 3. A typical signal to noise ratio was about 100 for the transition of J=26-25 in the ground vibrational state at 275 GHz. The spectral line intensity becomes weaker by about one half or one third as the vibrational quantum number increases by one. The intensity behavior with the vibrational quantum number suggests that the effective vibrational temperature of CaS is 600-900 K. Seven rotational transitions of MgS in the ground vibrational state were observed in the frequency region of 224-368 GHz, as listed in table 1. An example of the spectral lines observed is shown in fig. 2. The rotational constant and the centrifugal dis-
Table 1 Observed transition frequencies of MgS (D= 0) a) Transition J
v&a
Au b’
14-13 16-15 17-16 18-17 19-18 22-21 23-22
224103.178(45) c, 256086.116( 19) 272072.920(29) 288056.376(26) 304036.237( 29) 351952.415(30) 367915.973(43)
0.019 - 0.006 -0.017 0.000 -0.003 0.015 - 0.007
a) In MHz. b’ Av= vh - vcailc. ” The values in parentheses are one standard deviation of the frequency measurements and apply to the last digits of the frequencies.
564
I
I 224101
21 July 1989 I
I
224103
I
I
224105
4
FREQUENCYlMHz)
Pig. 2. The 3= 14-l 3 rotational spectral line of MgS. The integration time is about 25 s.
tortion constant were determined from the observed spectral lines by a least-squares method. The standard deviation of the fit was 0.014 MHz. The molecular constants determined are given in table 2. The molecular constants agree with those obtained from the electronic absorption spectrum [ 7 1, but the accuracy of the constants has been improved by two orders of magnitude. Eight to ten rotational transitions of CaS in the region of 198-306 GHz were observed for each vibrational state, as listed in table 3. The rotational constants and the centrifugal distortion constants determined by least-squares fits are shown in table 2. The standard deviations of the fits were 0,017, 0.025, 0.018 and 0.030 MHz for the v=O, 1, 2 and 3 states, respectively. They also agree well with those obtained from the electronic absorption spectrum
[61. The rotational constant B, is expressed in the power series expansion of V+ l/2. A least-squares fit of the rotational constants for CaS gave the equilibrium rotational constant and the vibration-rotation constants to be B,=5296.6032( 15), a,=24.7921(22), and y,=-0.06959(60) MHz, where the values in parentheses are three times the standard deviations of the fit. The equilibrium internuclear distance of CaS was derived to be 2.317751( 1) A. The error given in parentheses originates mainly from the er-
CHEMICAL
Volume 159, number $6 Table 2 Molecular
constants
of M@ and CaS a) MW b’
II
Optical
MBS
0
& &
8006.9278(15) 0.00X2744( 19)
CaS
0
Es
5284.1898(15) 0.0031021(11)
DO Ho 81 DI HI
2
& DZ Hz
5234.1881(18) 0.0031341(14)
Bs D3
5208.9781(29) 0.0031496(22)
8007.25(39) ‘) 0.008280 (30) ‘) 5283.87( 18) d, 0.003058( 12) d, -1.01(20)x10-~d’
-
1
3
21 July 1989
PHYSICS LETTERS
5258.66(24) d’ 0.0030609(90) d, -1.271(75)x10-96’
5259.2581(21) 0.0031182(16) -
5233.69(33) ‘) 0.003067 (30) ‘) -1.66(75)x 1O-9d’
-
-
*) In MHz. The values in parentheses are three standard deviations and apply to the last digits of the constants. b, This study. c) Ref. [ 71. The values, originally reported in cm-‘, are converted to MHz. d, Ref. [6]. The values, originally reported in cm-‘, are converted to MHz.
rors in the equilibrium rotational constant and the fundamental physical constants. The small contribution of electrons to the moment of inertia was not included in the calculation. The four centrifugal distortion constants were also fitted to the power series expansion of v+ l/2 as D,(MHz)=0.00309391(18)
~lnol.r.mb~\o-d-+oN400N
~ooooooooc 5ddddddddl
+0.00001644(26)(u+1/2) -0.000000149(71)(u+
I
l/2)2.
I
I
I
I
(1)
The first term is De, and the coeffkient of the second term A, The vibrational frequency W, and the anharmonicity constant WJ~ were calculated to be 462.3 and 1.634 cm- ’ , respectively, from B,, De and (Y, [ 9 1. They are consistent with those obtained from the electronic absorption spectrum [ 6 1. MgS could not be observed by the reaction of magnesium vapor with OCS. The heat of reaction of Mg+ OCS+MgS + CO in the gas phase is estimated to be endothermic by more than 80 kJ/mol. On the other hand, the reaction of magnesium vapor with S2 is exothermic. The heat of the three-body reaction 565
Volume 159, number
5,6
CHEMICAL
PHYSICS LIZ-l-I-ERS
2Mg+S,-+ZMgS in the gas phase is estimated to be exothermic by less than 38 kJ/mol. For CaS, which was readily produced by the reaction of calcium vapor with OCS, the heat of reaction is also estimated to be exothermic by about 23 kJ/mol. Therefore, these exothermic reactions are considered to be effective for the production of MgS and CaS. The molecular constants obtained in the present study enabled us to calculate the frequencies of lowJ transitions with an accuracy sufficient for the radioastronomical search for MgS and CaS. T’he spectral data toward relatively warm sources, our galactic center SgrB2 (0.080 K), the red giant star IRC + 102 16 (0.068 K), and the star-forming region W51A (0.052 K) [lo] for MgS (J=7-6), and IRC + 102 16 [ 111 for CaS (J= 7-6 ) were examined at the relevant frequencies calculated. The values in parentheses are root mean squares of the noise in antenna temperature. No lines corresponding to the transitions of MgS or CaS were found above the signal to noise ratio levels attained at present. We are grateful to N. Kaifu for the spectral line data obtained by the 45 m radiotelescope at Nobeyama Radio Observatory. We also thank the staff
566
21 July 1989
members of the Instrumental Development Center of Faculty of Science, Nagoya University for their construction of the cell used in this study. This study was supported by a Grant-in-Aid from the Ministry of Education, Science, and Culture (Nos. 62470015, 62740277,63632514, and 63540192).
References [ I] J. Cemicharo and M. GuClin, Astron. Astrophys. 183 ( 1987) LIO. [2] BE Tumer and T.C. Steimle, Astrophys. J. 299 (1985) 956. [3] T.J. M&r, J. EUd&, A. Hjahwson and H. Olofbson, Astron. Astrophys. 182 (1987) 143. [4] W.H. Hocking, G. Winnewisser, E. Churchwell and J. Percival, Astron. Astrophys. 75 (1979) 268. [ 5] T. Tsuji, Astron. Astrophys. 23 (1973) 411. [ 6 1 R.C. Blues and RF. E%arrow,Trans. Faraday Sot. 65 ( 1969 )
[ 71 ~?Marcanc~
and R.F. Barrow, Trans. Faraday Sot. 66 (1970) 2936. [S] S. Yamamoto and S. Saito, J. Chem. Phys. 89 (1988) 1936. [9] W. Gordy and R.L. Cook, Microwave molecular spectra, 3rd Ed. ( Wiley-Interscience, New York, 1984) ch. 4. [ lo] N. Kaifu, private communication. [ 111J. Cemicharo, C. Kahane, J. Gbmez-Gonzalez and M. Gu6li11, Astron. Astrophys. 164 (1986) Ll.