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Millimeter- and submillimeter-wave spectra of platinum monosulfide (PtS) in the X3Σ0- state
Journal of Molecular Spectroscopy 278 (2012) 7–10
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Millimeter- and submillimeter-wave spectra of platinum monosulfide (PtS) in the X 3 R 0 state Toshiaki Okabayashi a,⇑, Takuya Yamamoto b, Taku Kurahara b, Den-ichiro Mizuguchi b, Shohei Mizuno b, Mitsutoshi Tanimoto c a b c
Graduate School of Science and Technology, Shizuoka University, 836 Oya, Shizuoka 422-8529, Japan Department of Chemistry, Faculty of Science, Shizuoka University, 836 Oya, Shizuoka 422-8529, Japan Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka 259-1293, Japan
a r t i c l e
i n f o
Article history: Received 26 April 2012 In revised form 2 July 2012 Available online 16 July 2012 Keywords: Microwave spectrum Platinum monosulfide Sputtering reaction
a b s t r a c t The millimeter- and submillimeter-wave spectra of PtS in the X 3 R 0 state were observed by employing a source-modulated microwave spectrometer. The PtS radical was generated in a free space cell by using a dc glow plasma of H2S and Ar. Platinum atoms were supplied from a small piece of platinum sheet lining the inner surface of a stainless steel cathode by the sputtering reaction. Rotational transitions were recorded for four dominant isotopomers, 194PtS, 195PtS, 196PtS, and 198PtS in the X 3 R 0 ground state, and the spectroscopic constants for each isotopomer were determined using a Hund’s case (c) Hamiltonian. 3 The lower limit of the 3 R 0 R1 energy separation is estimated from a Hund’s case (a) analysis. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Group 10 metals (Ni, Pd, and Pt) and their compounds are widely used in various chemical processes as catalysts, such as three-way catalysts to remove CO, NOx , and hydrocarbons from car exhaust [1,2], and the desulfurization catalyst to produce sulfur-free fuel [3]. In such processes, oxygen or sulfur atoms in the reactants interact with metallic atoms on the catalyst surface. Thus, diatomic group-10-metal monochalcogenides attract interest as the simplest models for reactant-catalyst interactions. Hence, it is advantageous to carry out spectroscopic studies on such metal monochalcogenides to determine their physico-chemical properties. Platinum, the heaviest member of the group 10 metals, is well known as one of the most important catalytic metals, but its chalcogenide PtX species have not been comprehensively studied. On the other hand, the lightest member of the platinum monochalcogenides, PtO, has been rather well studied using spectroscopic methods [4–13]. For the next member, the monosulfide PtS, there are only a small number of spectroscopic studies. Laser-induced fluorescence spectroscopy was initially used to detect PtS produced by laser ablation in the gas phase in 1995 [14]. At the same time, rotational transitions from 44 to 71 GHz were observed with pump/probe microwave optical double resonance. The same group determined the electric dipole moment of PtS [15]. Fourier transform microwave spectroscopy was used to observe rotational transitions below 20 GHz [16]. The vibrational spectrum of PtS was ⇑ Corresponding author. Fax: +81 54 237 3384. E-mail address: [email protected] (T. Okabayashi). 0022-2852/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jms.2012.07.002
observed in an Ar gas matrix in 2009 [17]. Recently, the near-infrared electronic spectrum was measured using intracavity laser spectroscopy [18]. Although the electronic ground state of PtS is X ¼ 0þ for Hund’s case (c), there is some uncertainty about which of 3 R (X = 0 and 1) and 1 Rþ (X = 0) states for Hund’s case (a) results in the ground state. Even if the ground state of PtS is 3 R , its lowest spin compo1 þ nent 3 R 0 should be nearly isolated such as R , because the energy 3 3 separation between the R0 and R1 components is large as a result of the large atomic spin–orbit constant of Pt (f(Pt) = 4221 cm1 [19]). In fact, the oxygen analog PtO in the 3 R ground state has a large energy separation (937 cm1) between the two components [8], and its ground state was previously considered to be 1 Rþ . A similar spectroscopic feature was seen in the electronic spectrum of PtS, indicating that the substance acted as a well-behaved real singlet 0+ molecule [14]. As far as authors know, there is no theoretical calculation that mentions which of the 3 R and 1 Rþ states is the ground state. On the basis of the large and negative Born– Oppenheimer breakdown parameters of the ground 0+ state, Cooke and Gerry [16] indicated that the ground 0+ state are considerably affected by a low-lying 1+ state. This finding suggests that the elec1 þ tronic ground state of PtS is much more preferably 3 R 0 than R . However, neither direct nor indirect spectroscopic characterization of the 3 R 1 state has been reported. If PtS possesses to some extent Hund’s case (a) character, the 3 R1 substate might disturb the case (c) molecular constants of the 3 R 0 substate. For an ideal case (c) X ¼ 0 molecule, the rotational energy well satisfies the J n ðJ þ 1Þn power series. However, this correlation should be somewhat less effectively expressed
8
T. Okabayashi et al. / Journal of Molecular Spectroscopy 278 (2012) 7–10
when the rotational energy in a case (a) 3 R 0 molecule is fitted to the J n ðJ þ 1Þn power series. Thus, if the molecule has some degree of case (a) 3 R 0 character, higher order centrifugal distortion constants will be noticeable in the case (c) analysis. This type of contribution becomes much clearer when higher-J-value transitions are measured. However, this effect is so small that a spectroscopic method with substantially high resolution would be required to detect it. In this context, highly accurate microwave spectroscopy may distinguish such a small contribution from experimental error. In this paper, we report an extension of the rotational measurement of PtS up to 406 GHz. The PtS radical was generated from a platinum sheet on the cathode using a sputtering technique. Rotational transitions were recorded for four dominant isotopomers, 194 PtS, 195PtS, 196PtS, and 198PtS in the X 3 R 0 ground state, and the spectroscopic constants were determined for each isotopomer using a Hund’s case (c) Hamiltonian. The lower limit of the 3 R0 3 R1 energy separation is estimated from a Hund’s case (a) analysis.
195 3
291006
(MHz)
Fig. 1. Rotational transition of
PtS
Σ0
−
J=33−32
291012
195
PtS in the X 3 R 0 ground state.
2. Experimental The present experiment was carried out using a source-modulated microwave spectrometer [20]. Millimeter- and submillimeter-wave radiations were generated by frequency-multipliers driven by a series of klystrons. The radiation transmitted through a free space cell was detected by an InSb bolometer cooled by liquid helium. The cell was equipped with a pair of cylindrical electrodes for dc glow discharge and was covered by a cooling jacket made of copper through which liquid nitrogen was circulated. To cancel the magnetic field of earth, electromagnetic coils were wound outside of the cell. Platinum atoms for PtS production were supplied by sputtering a small piece of a platinum sheet lining the inner surface of a stainless steel cathode. A similar reaction has been employed in microwave studies of transition metal monosulfides, FeS [21] and NiS [22], as well as platinum compounds, PtO [12], PtCO [23], PtCN [24], PtF [25], and PtCl [25]. The transition frequencies of PtS in the ground state were predicted using the molecular constants determined from the Fourier transform microwave spectrum [16]. The rotational transitions were readily observed near the predicted frequencies (within 1 MHz) for the four dominant isotopomers, 194PtS, 195PtS, 196PtS, and 198PtS in the X 3 R 0 state. Sample pressures were optimized to a trace amount of H2S diluted with 4 mTorr of Ar. The discharge current was set to approximately 300 mA. The cell was maintained near 150 °C for efficient PtS generation. Under the optimal conditions, we searched for transitions in the higher X 3 R 1 spin substate but detected no candidate lines because of the poor signal-to-noise ratio. The spectrum in the first excited vibrational state, which was located 491 cm1 above the ground vibrational state [17], was also probed without success. In total, 52 lines of PtS in the X 3 R 0 ground vibronic state were observed between 149 and 406 GHz. Fig. 1 displays a typical PtS spectral line. 3. Analysis The observed transition frequencies for each isotopomer were independently analyzed using the standard polynomial expression of a Hund’s case (c) coupling scheme,
Ev ;J ¼ Bv ½JðJ þ 1Þ Dv ½JðJ þ 1Þ2 þ Hv ½JðJ þ 1Þ3 :
ð1Þ
Analysis of the present millimeter and submillimeter data combined with microwave data [14,16] led to the molecular constants listed in Table 1. The observed rotational transition frequencies
and residuals of the fit are summarized in Table 2. The standard deviation of the fit for each isotopomer is approximately 11– 22 kHz, which is reasonable in view of the expected measurement error. The sextic centrifugal distortion constant H0 is required for the fit converging within the experimental accuracy when rotational transitions up to J ¼ 46 45 are included in the analysis. 4. Discussion The sextic centrifugal distortion constant (H) for a diatomic molecule is given by the formula [26]
H0 ’ He ¼
2De ð12B2e xe ae Þ: 3x2e
ð2Þ
Using the experimental values given in Ref. [16], He is estimated to be 0.234 mHz, which is in good agreement with the experimental H0 values, ranging from 0.225 to 0.336 mHz. The present H values are close to that of AuCl (0.264 mHz) [27], which is a simple 1 Rþ diatomic molecule having a reduced mass similar to that of PtS. Since AuCl has a closed-shell electronic structure without lowlying electronic excited states, the H constant should originate from the anharmonic vibrational potential terms. These facts suggest a 3 large energy separation between the R 0 and R1 substates of PtS. 3 There is no evidence of an apparent R0 3 R 1 perturbation in the rotational spectrum of PtS. We estimate the lower limit of the energy separation using a Hund’s case (a) effective Hamiltonian [28]. In this analysis the sextic centrifugal distortion constant of PtS under the perturbation-free conditions is assumed to be 0.264 mHz (the value of AuCl) [27]. Since the spin-rotation constant (c) is proportional to the rotational constant, c(PtS) is estimated as follows: cðPtSÞ ’ cðPtOÞ B0 ðPtSÞ=B0 ðPtOÞ ’ 3600 MHz. To make this assumption valid, we assumed that the electronic configurations of PtO and PtS are not drastically different and that their c=B0 values do not vary significantly. The residuals of the least squares fit under these assumptions are illustrated in Fig. 2. For c ’ 3600 MHz, the residuals rapidly increase when the spin–spin constant (k) is below 5 THz, where 3 2k represents the energy separation between the R R1 0 and substates in the case (a) limit. Thus, this value is approximated as the lower limit of k. If the assumed c is half or double the value, the lower limits of k still fall to 4 or 7 THz, neither of which is drastically different from k ’ 5 THz. Therefore, we think that the lower limit of the energy separation for PtS is reasonably estimated as 2k ’ 10 THz (300 cm1). Cooke et al. roughly estimated the
9
T. Okabayashi et al. / Journal of Molecular Spectroscopy 278 (2012) 7–10 Table 1 Molecular constants for PtS.a 194
195
PtS
196
PtS
198
PtS
PtS
This study B0 /MHz D0 /kHz H0 /mHz
4415.57580(12) 1.44473(13) 0.225(36)
4412.37039(19) 1.44251(18) 0.251(47)
4409.20337(16) 1.44011(17) 0.336(47)
4402.95813(37) 1.43608(21) 0.336b
FTMWc B0 /MHz D0 /kHz
4415.57588d 1.449d
4412.37043(99) 1.440(136)
4409.20382d 1.445d
4402.95856d 1.441d
a
Values in parentheses represent one standard deviation. Fixed in the analysis. From [16]. Derived from mass-independent Dunham constants.
b c d
Table 2 Observed transition frequencies for PtS in MHz. 194
195
PtS
0
J J
00
1–0 2–1 5–4 6–5 8–7 17–16 18–17 19–18 22–21 23–22 26–25 27–26 28–27 33–32 34–33 35–34 43–42 44–43 45–44 46–45 a b c d e
196
PtS
a
198
PtS
PtS
Obs. freq.
O–C
Obs. freq.
O–C
Obs. freq.
O–C
Obs. freq.
O–Ca
8831.1458 17662.2578 44155.0373 52985.6640 70646.2476 150101.174 158927.014 167752.235 194223.817 203046.114 229508.357 238327.338 247145.362 291220.279 300031.943 308842.459 379279.896 388078.183 396874.960 405670.201
0.0000b 0.0008b 0.0017cd 0.0027c,d 0.0063c,d 0.009 0.009 0.004 0.023 0.052e 0.002 0.011 0.000 0.007 0.015 0.005 0.041e 0.008 0.008 0.003
8824.7351 17649.4357 44122.9370 52947.1850 70594.9310 149992.228 158811.679 167630.483 194082.866 202898.902 229341.833 238154.413 246966.057 291009.030 299814.346 308618.437 379004.878 387796.816 396587.256 405376.148
0.0001b 0.0003b 0.0456c,e 0.0133c 0.0409c,e 0.015 0.001 0.011 0.016 0.078e 0.005 0.005 0.005 0.001 0.014 0.021 0.005 0.015 0.006 0.015
8818.4021 17636.7694 44091.3090 52909.1940 70544.3010 149884.614 158697.736 167510.204 193943.555 202753.258 229177.318 237983.566 246788.896 290800.322 299599.336 308397.142 378733.200 387518.898 396302.983 405085.597
0.0011b 0.0020b 0.0047c,d 0.0022c,d 0.0036c,d 0.003 0.013 0.008 0.046 e 0.003 0.012 0.004 0.005 0.009 0.007 0.009 0.002 0.032 0.028 0.001
8805.9112 17611.7885
0.0007b 0.0020b
167272.998
0.005
228852.832 237646.617 246439.516 290388.706 299175.320 307960.997
0.004 0.027 0.005 0.017 0.035 0.323e
Observed minus calculated frequencies. From FTMW spectroscopy [16]. Frequencies of From MODR spectroscopy [14]. Weighted 10.0. Excluded from the fit.
Residual (MHz)
0.1
PtS are calculated values without hyperfine splitting. Weighted 100.0.
0.1
H = −0.264 mHz γ = −1.8 GHz
H = −0.264 mHz γ = −3.6 GHz
0.05
0
a
195
0.05
0
a
5
0
10
0.1
H =−0.264 mHz γ = −7.2 GHz
0.05
0
λ (THz)
5
10
0
0
5
194
PtS
195
PtS
10
λ (THz)
λ (THz) 196
PtS
Fig. 2. Residuals in the Hund’s case (a) analysis. Arrows in the figure indicate the estimated lower limits of k.
separation for PtS to be 471 cm1 from the Born–Oppenheimer breakdown [16]. Although they suggested that this value was likely to be too small, their estimate is qualitatively consistent with the present lower limit. As the oxygen analog PtO has the large energy separation of 937 cm1 [8], it is quite possible that the actual c(PtS) is much larger than our lower limit. This assumption reasonably
explains the fact that the 3 R 1 component was not observed in the glow discharge. The energy separation for PtS is also much larger than those for the lightest members of the group 10 metal monochalcogenides, nickel monoxide (NiO, 50 cm1 ) [29] and nickel monosulfide (NiS, 72 cm1 ) [22]. This is mainly caused by the difference
10
T. Okabayashi et al. / Journal of Molecular Spectroscopy 278 (2012) 7–10
between the atomic spin–orbit constants of platinum and nickel (fðPtÞ ¼ 4221 and fðNiÞ ¼ 663 cm1 , respectively) [19]. The spin– spin interactions of a molecule containing heavy atoms mainly arise from second order spin–orbit interactions with other states [19]. Thus k should be roughly proportional to f2 , and PtO and PtS have much larger k values than NiO and NiS. Acknowledgment This study was supported by the Japan Society for the Promotion of Science through a Grant-in-Aid for Scientific Research (Nos. 15656184, 20550010, and 23550014). References [1] [2] [3] [4] [5] [6] [7] [8]
K.C. Taylor, Catal. Rev. Sci. Eng. 35 (1993) 457. M. Shelef, G.W. Graham, Catal. Rev. Sci. Eng. 36 (1994) 433. A.N. Startsev, Catal. Rev. Sci. Eng. 37 (1995) 353. C. Nilsson, R. Scullman, N. Mehendale, J. Mol. Spectrosc. 35 (1970) 177. R. Scullman, U. Sassenberg, C. Nilsson, Can. J. Phys. 53 (1975) 1991. K. Jansson, R. Scullman, J. Mol. Spectrosc. 61 (1976) 299. U. Sassenberg, R. Scullman, J. Mol. Spectrosc. 68 (1977) 331. U. Sassenberg, R. Scullman, Phys. Scr. 28 (1983) 139.
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
C.I. Frum, R. Engleman Jr., P.F. Bernath, J. Mol. Spectrosc. 150 (1991) 566. T.C. Steimle, K.Y. Jung, B.-Z. Li, J. Chem. Phys. 103 (1995) 1767. H. Liu, L.C. O’Brien, S. Shaji, J.J. O’Brien, J. Mol. Spectrosc. 253 (2009) 73. T. Okabayashi, E. Yamazaki, M. Tanimoto, J. Mol. Spectrosc. 229 (2005) 283. S.A. Cooke, M.C.L. Gerry, Phys. Chem. Chem. Phys. 7 (2005) 2453. B.-Z. Li, K.Y. Jung, T.C. Steimle, J. Mol. Spectrosc. 170 (1995) 310. T.C. Steimle, K.Y. Jung, B.-Z. Li, J. Chem. Phys. 103 (1995) 1767. S.A. Cooke, M.C.L. Gerry, J. Chem. Phys. 121 (2004) 3486. B. Liang, X. Wang, L. Andrews, J. Phys. Chem. A 113 (2009) 3336. K. Handler, L.C. O’Brien, J.J. O’Brien, J. Mol. Spectrosc. 263 (2010) 78. H. Lefebvre-Brion, R.W. Field, The Spectra and Dynamics of Diatomic Molecules, Elsevier, San Diego, 2004. T. Okabayashi, M. Tanimoto, J. Chem. Phys. 99 (1993) 3268. S. Takano, S. Yamamoto, S. Saito, J. Mol. Spectrosc. 224 (2004) 137. T. Yamamoto, M. Tanimoto, T. Okabayashi, Phys. Chem. Chem. Phys. 9 (2007) 3744. E. Yamazaki, T. Okabayashi, M. Tanimoto, Chem. Phys. Lett. 396 (2004) 150. E.Y. Okabayashi, T. Okabayashi, T. Furuya, M. Tanimoto, Chem. Phys. Lett. 492 (2010) 25. T. Okabayashi, T. Kurahara, E.Y. Okabayashi, M. Tanimoto, J. Chem. Phys. 136 (2012) 174311. W. Gordy, R.L. Cook, Microwave Molecular Spectra, third ed., Wiley, New York, 1984. T. Okabayashi, E. Yamazaki, K. Tsukamoto, M. Tanimoto, J. Mol. Spectrosc. 220 (2003) 155. C.R. Brazier, R.S. Ram, P.F. Bernath, J. Mol. Spectrosc. 120 (1986) 381. R.S. Ram, P.F. Bernath, J. Mol. Spectrosc. 155 (1992) 315.
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Note
Millimeter- and submillimeter-wave spectra of platinum monosulfide (PtS) in the X 3 R 0 state Toshiaki Okabayashi a,⇑, Takuya Yamamoto b, Taku Kurahara b, Den-ichiro Mizuguchi b, Shohei Mizuno b, Mitsutoshi Tanimoto c a b c
Graduate School of Science and Technology, Shizuoka University, 836 Oya, Shizuoka 422-8529, Japan Department of Chemistry, Faculty of Science, Shizuoka University, 836 Oya, Shizuoka 422-8529, Japan Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka 259-1293, Japan
a r t i c l e
i n f o
Article history: Received 26 April 2012 In revised form 2 July 2012 Available online 16 July 2012 Keywords: Microwave spectrum Platinum monosulfide Sputtering reaction
a b s t r a c t The millimeter- and submillimeter-wave spectra of PtS in the X 3 R 0 state were observed by employing a source-modulated microwave spectrometer. The PtS radical was generated in a free space cell by using a dc glow plasma of H2S and Ar. Platinum atoms were supplied from a small piece of platinum sheet lining the inner surface of a stainless steel cathode by the sputtering reaction. Rotational transitions were recorded for four dominant isotopomers, 194PtS, 195PtS, 196PtS, and 198PtS in the X 3 R 0 ground state, and the spectroscopic constants for each isotopomer were determined using a Hund’s case (c) Hamiltonian. 3 The lower limit of the 3 R 0 R1 energy separation is estimated from a Hund’s case (a) analysis. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Group 10 metals (Ni, Pd, and Pt) and their compounds are widely used in various chemical processes as catalysts, such as three-way catalysts to remove CO, NOx , and hydrocarbons from car exhaust [1,2], and the desulfurization catalyst to produce sulfur-free fuel [3]. In such processes, oxygen or sulfur atoms in the reactants interact with metallic atoms on the catalyst surface. Thus, diatomic group-10-metal monochalcogenides attract interest as the simplest models for reactant-catalyst interactions. Hence, it is advantageous to carry out spectroscopic studies on such metal monochalcogenides to determine their physico-chemical properties. Platinum, the heaviest member of the group 10 metals, is well known as one of the most important catalytic metals, but its chalcogenide PtX species have not been comprehensively studied. On the other hand, the lightest member of the platinum monochalcogenides, PtO, has been rather well studied using spectroscopic methods [4–13]. For the next member, the monosulfide PtS, there are only a small number of spectroscopic studies. Laser-induced fluorescence spectroscopy was initially used to detect PtS produced by laser ablation in the gas phase in 1995 [14]. At the same time, rotational transitions from 44 to 71 GHz were observed with pump/probe microwave optical double resonance. The same group determined the electric dipole moment of PtS [15]. Fourier transform microwave spectroscopy was used to observe rotational transitions below 20 GHz [16]. The vibrational spectrum of PtS was ⇑ Corresponding author. Fax: +81 54 237 3384. E-mail address: [email protected] (T. Okabayashi). 0022-2852/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jms.2012.07.002
observed in an Ar gas matrix in 2009 [17]. Recently, the near-infrared electronic spectrum was measured using intracavity laser spectroscopy [18]. Although the electronic ground state of PtS is X ¼ 0þ for Hund’s case (c), there is some uncertainty about which of 3 R (X = 0 and 1) and 1 Rþ (X = 0) states for Hund’s case (a) results in the ground state. Even if the ground state of PtS is 3 R , its lowest spin compo1 þ nent 3 R 0 should be nearly isolated such as R , because the energy 3 3 separation between the R0 and R1 components is large as a result of the large atomic spin–orbit constant of Pt (f(Pt) = 4221 cm1 [19]). In fact, the oxygen analog PtO in the 3 R ground state has a large energy separation (937 cm1) between the two components [8], and its ground state was previously considered to be 1 Rþ . A similar spectroscopic feature was seen in the electronic spectrum of PtS, indicating that the substance acted as a well-behaved real singlet 0+ molecule [14]. As far as authors know, there is no theoretical calculation that mentions which of the 3 R and 1 Rþ states is the ground state. On the basis of the large and negative Born– Oppenheimer breakdown parameters of the ground 0+ state, Cooke and Gerry [16] indicated that the ground 0+ state are considerably affected by a low-lying 1+ state. This finding suggests that the elec1 þ tronic ground state of PtS is much more preferably 3 R 0 than R . However, neither direct nor indirect spectroscopic characterization of the 3 R 1 state has been reported. If PtS possesses to some extent Hund’s case (a) character, the 3 R1 substate might disturb the case (c) molecular constants of the 3 R 0 substate. For an ideal case (c) X ¼ 0 molecule, the rotational energy well satisfies the J n ðJ þ 1Þn power series. However, this correlation should be somewhat less effectively expressed
8
T. Okabayashi et al. / Journal of Molecular Spectroscopy 278 (2012) 7–10
when the rotational energy in a case (a) 3 R 0 molecule is fitted to the J n ðJ þ 1Þn power series. Thus, if the molecule has some degree of case (a) 3 R 0 character, higher order centrifugal distortion constants will be noticeable in the case (c) analysis. This type of contribution becomes much clearer when higher-J-value transitions are measured. However, this effect is so small that a spectroscopic method with substantially high resolution would be required to detect it. In this context, highly accurate microwave spectroscopy may distinguish such a small contribution from experimental error. In this paper, we report an extension of the rotational measurement of PtS up to 406 GHz. The PtS radical was generated from a platinum sheet on the cathode using a sputtering technique. Rotational transitions were recorded for four dominant isotopomers, 194 PtS, 195PtS, 196PtS, and 198PtS in the X 3 R 0 ground state, and the spectroscopic constants were determined for each isotopomer using a Hund’s case (c) Hamiltonian. The lower limit of the 3 R0 3 R1 energy separation is estimated from a Hund’s case (a) analysis.
195 3
291006
(MHz)
Fig. 1. Rotational transition of
PtS
Σ0
−
J=33−32
291012
195
PtS in the X 3 R 0 ground state.
2. Experimental The present experiment was carried out using a source-modulated microwave spectrometer [20]. Millimeter- and submillimeter-wave radiations were generated by frequency-multipliers driven by a series of klystrons. The radiation transmitted through a free space cell was detected by an InSb bolometer cooled by liquid helium. The cell was equipped with a pair of cylindrical electrodes for dc glow discharge and was covered by a cooling jacket made of copper through which liquid nitrogen was circulated. To cancel the magnetic field of earth, electromagnetic coils were wound outside of the cell. Platinum atoms for PtS production were supplied by sputtering a small piece of a platinum sheet lining the inner surface of a stainless steel cathode. A similar reaction has been employed in microwave studies of transition metal monosulfides, FeS [21] and NiS [22], as well as platinum compounds, PtO [12], PtCO [23], PtCN [24], PtF [25], and PtCl [25]. The transition frequencies of PtS in the ground state were predicted using the molecular constants determined from the Fourier transform microwave spectrum [16]. The rotational transitions were readily observed near the predicted frequencies (within 1 MHz) for the four dominant isotopomers, 194PtS, 195PtS, 196PtS, and 198PtS in the X 3 R 0 state. Sample pressures were optimized to a trace amount of H2S diluted with 4 mTorr of Ar. The discharge current was set to approximately 300 mA. The cell was maintained near 150 °C for efficient PtS generation. Under the optimal conditions, we searched for transitions in the higher X 3 R 1 spin substate but detected no candidate lines because of the poor signal-to-noise ratio. The spectrum in the first excited vibrational state, which was located 491 cm1 above the ground vibrational state [17], was also probed without success. In total, 52 lines of PtS in the X 3 R 0 ground vibronic state were observed between 149 and 406 GHz. Fig. 1 displays a typical PtS spectral line. 3. Analysis The observed transition frequencies for each isotopomer were independently analyzed using the standard polynomial expression of a Hund’s case (c) coupling scheme,
Ev ;J ¼ Bv ½JðJ þ 1Þ Dv ½JðJ þ 1Þ2 þ Hv ½JðJ þ 1Þ3 :
ð1Þ
Analysis of the present millimeter and submillimeter data combined with microwave data [14,16] led to the molecular constants listed in Table 1. The observed rotational transition frequencies
and residuals of the fit are summarized in Table 2. The standard deviation of the fit for each isotopomer is approximately 11– 22 kHz, which is reasonable in view of the expected measurement error. The sextic centrifugal distortion constant H0 is required for the fit converging within the experimental accuracy when rotational transitions up to J ¼ 46 45 are included in the analysis. 4. Discussion The sextic centrifugal distortion constant (H) for a diatomic molecule is given by the formula [26]
H0 ’ He ¼
2De ð12B2e xe ae Þ: 3x2e
ð2Þ
Using the experimental values given in Ref. [16], He is estimated to be 0.234 mHz, which is in good agreement with the experimental H0 values, ranging from 0.225 to 0.336 mHz. The present H values are close to that of AuCl (0.264 mHz) [27], which is a simple 1 Rþ diatomic molecule having a reduced mass similar to that of PtS. Since AuCl has a closed-shell electronic structure without lowlying electronic excited states, the H constant should originate from the anharmonic vibrational potential terms. These facts suggest a 3 large energy separation between the R 0 and R1 substates of PtS. 3 There is no evidence of an apparent R0 3 R 1 perturbation in the rotational spectrum of PtS. We estimate the lower limit of the energy separation using a Hund’s case (a) effective Hamiltonian [28]. In this analysis the sextic centrifugal distortion constant of PtS under the perturbation-free conditions is assumed to be 0.264 mHz (the value of AuCl) [27]. Since the spin-rotation constant (c) is proportional to the rotational constant, c(PtS) is estimated as follows: cðPtSÞ ’ cðPtOÞ B0 ðPtSÞ=B0 ðPtOÞ ’ 3600 MHz. To make this assumption valid, we assumed that the electronic configurations of PtO and PtS are not drastically different and that their c=B0 values do not vary significantly. The residuals of the least squares fit under these assumptions are illustrated in Fig. 2. For c ’ 3600 MHz, the residuals rapidly increase when the spin–spin constant (k) is below 5 THz, where 3 2k represents the energy separation between the R R1 0 and substates in the case (a) limit. Thus, this value is approximated as the lower limit of k. If the assumed c is half or double the value, the lower limits of k still fall to 4 or 7 THz, neither of which is drastically different from k ’ 5 THz. Therefore, we think that the lower limit of the energy separation for PtS is reasonably estimated as 2k ’ 10 THz (300 cm1). Cooke et al. roughly estimated the
9
T. Okabayashi et al. / Journal of Molecular Spectroscopy 278 (2012) 7–10 Table 1 Molecular constants for PtS.a 194
195
PtS
196
PtS
198
PtS
PtS
This study B0 /MHz D0 /kHz H0 /mHz
4415.57580(12) 1.44473(13) 0.225(36)
4412.37039(19) 1.44251(18) 0.251(47)
4409.20337(16) 1.44011(17) 0.336(47)
4402.95813(37) 1.43608(21) 0.336b
FTMWc B0 /MHz D0 /kHz
4415.57588d 1.449d
4412.37043(99) 1.440(136)
4409.20382d 1.445d
4402.95856d 1.441d
a
Values in parentheses represent one standard deviation. Fixed in the analysis. From [16]. Derived from mass-independent Dunham constants.
b c d
Table 2 Observed transition frequencies for PtS in MHz. 194
195
PtS
0
J J
00
1–0 2–1 5–4 6–5 8–7 17–16 18–17 19–18 22–21 23–22 26–25 27–26 28–27 33–32 34–33 35–34 43–42 44–43 45–44 46–45 a b c d e
196
PtS
a
198
PtS
PtS
Obs. freq.
O–C
Obs. freq.
O–C
Obs. freq.
O–C
Obs. freq.
O–Ca
8831.1458 17662.2578 44155.0373 52985.6640 70646.2476 150101.174 158927.014 167752.235 194223.817 203046.114 229508.357 238327.338 247145.362 291220.279 300031.943 308842.459 379279.896 388078.183 396874.960 405670.201
0.0000b 0.0008b 0.0017cd 0.0027c,d 0.0063c,d 0.009 0.009 0.004 0.023 0.052e 0.002 0.011 0.000 0.007 0.015 0.005 0.041e 0.008 0.008 0.003
8824.7351 17649.4357 44122.9370 52947.1850 70594.9310 149992.228 158811.679 167630.483 194082.866 202898.902 229341.833 238154.413 246966.057 291009.030 299814.346 308618.437 379004.878 387796.816 396587.256 405376.148
0.0001b 0.0003b 0.0456c,e 0.0133c 0.0409c,e 0.015 0.001 0.011 0.016 0.078e 0.005 0.005 0.005 0.001 0.014 0.021 0.005 0.015 0.006 0.015
8818.4021 17636.7694 44091.3090 52909.1940 70544.3010 149884.614 158697.736 167510.204 193943.555 202753.258 229177.318 237983.566 246788.896 290800.322 299599.336 308397.142 378733.200 387518.898 396302.983 405085.597
0.0011b 0.0020b 0.0047c,d 0.0022c,d 0.0036c,d 0.003 0.013 0.008 0.046 e 0.003 0.012 0.004 0.005 0.009 0.007 0.009 0.002 0.032 0.028 0.001
8805.9112 17611.7885
0.0007b 0.0020b
167272.998
0.005
228852.832 237646.617 246439.516 290388.706 299175.320 307960.997
0.004 0.027 0.005 0.017 0.035 0.323e
Observed minus calculated frequencies. From FTMW spectroscopy [16]. Frequencies of From MODR spectroscopy [14]. Weighted 10.0. Excluded from the fit.
Residual (MHz)
0.1
PtS are calculated values without hyperfine splitting. Weighted 100.0.
0.1
H = −0.264 mHz γ = −1.8 GHz
H = −0.264 mHz γ = −3.6 GHz
0.05
0
a
195
0.05
0
a
5
0
10
0.1
H =−0.264 mHz γ = −7.2 GHz
0.05
0
λ (THz)
5
10
0
0
5
194
PtS
195
PtS
10
λ (THz)
λ (THz) 196
PtS
Fig. 2. Residuals in the Hund’s case (a) analysis. Arrows in the figure indicate the estimated lower limits of k.
separation for PtS to be 471 cm1 from the Born–Oppenheimer breakdown [16]. Although they suggested that this value was likely to be too small, their estimate is qualitatively consistent with the present lower limit. As the oxygen analog PtO has the large energy separation of 937 cm1 [8], it is quite possible that the actual c(PtS) is much larger than our lower limit. This assumption reasonably
explains the fact that the 3 R 1 component was not observed in the glow discharge. The energy separation for PtS is also much larger than those for the lightest members of the group 10 metal monochalcogenides, nickel monoxide (NiO, 50 cm1 ) [29] and nickel monosulfide (NiS, 72 cm1 ) [22]. This is mainly caused by the difference
10
T. Okabayashi et al. / Journal of Molecular Spectroscopy 278 (2012) 7–10
between the atomic spin–orbit constants of platinum and nickel (fðPtÞ ¼ 4221 and fðNiÞ ¼ 663 cm1 , respectively) [19]. The spin– spin interactions of a molecule containing heavy atoms mainly arise from second order spin–orbit interactions with other states [19]. Thus k should be roughly proportional to f2 , and PtO and PtS have much larger k values than NiO and NiS. Acknowledgment This study was supported by the Japan Society for the Promotion of Science through a Grant-in-Aid for Scientific Research (Nos. 15656184, 20550010, and 23550014). References [1] [2] [3] [4] [5] [6] [7] [8]
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