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Sub-doppler infrared absorption spectroscopy of Ar-HF [(1000) ← (0000)] in a linear supersonic jet
Volume 127, number 4
CHEMICAL PHYSICS LETTERS
ZOJune1986
SUB-DOPPLER INFRARED ABSORPTION SPECTROSCOPY OF Ar-HF I(lO’O) + (oO”O)I IN A LINEAR SUPERSONIC JET Christopher M. LOVETOY, Michael D. SCHUDER and David J. NESBIIT
1
Joint Instituiefor Laboratory Astrophysrcs, Uniuersiiy of Colorado and National Bureau of Standnrdr and Departmenl of Chemistry, Unruersrty of Colorado, Boulder, CO 80309, USA
Received 24 February 1986; in final form 4 April 1986
Ultra-sensitive tunable difference frequency IR absorption spectroscopy in a slit supersonic jet has been used to observe sub-Doppler spectra of Ar-HF in the (lO*O) HF stretch and (11’0) HF stretch plus van der Waals bend modes. Linewidths yield a lower limit of 3 X lo-’ s for the predissociation lifetime in the vibrationally metastable upper state. The sensitivity of these direct absorption methods ( s 2 X lo9 molecules/cm3 per quantum state), in conjunction with the wide tunability of the difference frequency laser (2.2-4.2 pm) permit high-resolution studies of a large class of van der Waals complexes.
Infrared studies of weakly bound van der Waals molecules provide considerable insight into the structure of non-rigid molecules, as well as the dynamics of vibrational predissociation, collisional energy transfer, and intramolecular vibrational redistribution phenomena [l-3]. Interest in small molecular species has been particularly keen due yo the feasibility of accurate theoretical treatments [4,5]. The elegant work of McKellar, Welsh and co-workers on inert gashydrogen complexes took advantage of the infrared transparency of the uncomplexed hydrogen to enable studies in a high pressure, multiple pass cell [6]. pine and co-workers have subsequently utilized Dopplerlimited tunable difference frequency methods to probe hydrogen fluoride and hydrogen chloride dimers (and isotopic variants) in a cooled White cell [7]. Quite recently, these methods have been used to observe inert gas-hydrogen chloride complexes in a 127 K, 72 m optical path length [ 81. Low temperature, supersonic molecular beam experiments offer the possibility of dramatically reducing spectral congestion (both from monomer and van der Waals species) as well as generating a wide variety of weakly bound species not accessible in sufficient concentrations under equilibrium conditions [9]. Such beam studies, however, require exceptional sensitivity ’ Quantum Phystcs Dtvision, National Bureau of Standards.
374
to compensate for the five orders of magnitude loss in path length between a White cell and a conventional pinhole supersonic expansion. In this communication we report the first high resolution, near infrared absorption spectrum of Ar-HF, using tunable difference frequency laser absorption spectroscopy in a supersonic slit expansion. IR absorption by Ar-HF is monitored by double passing a tunable difference frequency laser across a pulsed slit expansion of our own design based on a solenoid activated plunger. The gas pulse has rise and fall times s 100 ps. The slit dimensions are 0.5” by 0.005”; rotational temperatures obtained for ArHF from a 400 Torr (1 Torr = 130 Pa) reservoir (1% HFfAr) are typically 5 10 K at 1.5 cm from the nozzle (see fig. 1). The net 1” path length represents two orders of magnitude enhancement over conventional pinhole expansions. The velocity distribution along the slit dimension and petpendicuZar to the flow is effectively quenched by collisions with adjacent molecules; residual Doppler broadening is reduced to less than the IR laser frequency jitter (k2.5 MHz). The net result is a naturally collimated jet for sub-Doppler spectroscopy without loss of number density X path length. Details of the slit pulsed valve, as well as a full description of the experimental apparatus, will be presented in a later publication. A total of 26 near infrared transitions (P( 12)0 ~9-2614/86/$03.50 0 Elsevier Science Publishers B.V. (Norht-Holland Physics publishing Division)
CHEMICAL PHYSICS LETTERS
Volume 127, number 4
2OJune1986
Ar-HF 10’0) + (00’0) T=B’K
Ar-HF(
IODINE
ETALON 10
lO*O+OO*O)
REFERENCE
FRINGES
-267GHz-
---‘^ 3151.”
WAVENUMBERS
Fig. 1. Stick plot of the raw absorption data for the ur stretch of Ar-HF. The fluctuations are due to variations in IR laser intensity during the scan.
R(13)), in AI-HF have been observed in the vl high frequency stretching mode. A stick plot of the raw absorption data is shown in fig. 1. Fluctuations are due to variations in the IR laser intensity over the duration of the scan. IR transition frequencies are obtained to +O.OOl cm-l uncertainty by measuring both ArI+and dye laser frequencies with a traveling corner cube interferometer [lo] referenced to a polarization stabilized HeNe laser. Absolute frequency calibration is obtained from nearby HF monomer lines [ 1 I]. Typical signal-to-noise in our data is demonstrated in fig. 2 for four pulses signal averaged with a 500 ps boxcar gate (20 Hz.repetition rate, 2.5 min scan time). The ground vibrational state of Ar-HF is known to have a linear equilibrium geometry, with large amplitude zero-point motion in the bending coordinate [ 121. The v1 fundamental spectrum, therefore, will exhibit a simple P and R branch structure as is observed. The null gap between R(0) and P( 1) permits unambiguous J assignment of the lines. Transition frequencies have been fitted to a third-degree polynomial expansion in J(J t 1). The constants obtained are reported in table 1 along with their statistical uncertainties; agreement between the microwave [ 121 and the present near IR data for the ground state constants is seen to be excellent. The rotationless transition frequency in Ar-HF is approximately 10 cm-l red-shifted from the corresponding free HF transition. This can be interpreted
3952600
3952 500
3952 400
I 3952 300
I 3952 2ocl
IR FREQUENCY (cm-‘)
Fig. 2. Sample data scan through the R(3) + R(1) lines of Ar-HF. The upper trace is the absorbance signal averaged over four pulses of the valve with a 500 PCS boxcar gate. Peak absorption is estimated to be 0.2%; noise limit corresponds to roughly lO+ absorbance. The lower two traces are iodine fluorescence and etalon frequency markers for wavelength calibration purposes. The Ar-HF sub-Doppler linewidths are approximately 50 MHz fwhm. limited primarily by frequency jitter (*25 MHz)in the dye laser. Supersonic jet rotational temperatures of 5 10 K are readily achieved in this slit design with stagnation pressures of 400 Torr.
as a weakening of the I-IF bond upon complexation, or equivalently described as due to a greater attraction of Ar for vibrationally excited HF. Additional support for this latter perspective is evidenced in the positive AB for Ar-HF in the u1 = 1 state, i.e. the van der Waals bond contracts slightly upon excitation, even Table 1 Ar-HF rotational constantsa) I(10’0) + (OO”O)I This work (cm-‘) “0
Boo”o
h”o &o”o Dlooo 4lOOO
HlOOO Blo”o - Bw”o
-
3951.770(3) 0.102246(15) 2.34(9) x 106 0.102600(15) 2.04(8) x 1O-e 2.5(16) x lo-to 3.2(14) x lo-lo 3.9(9) x 10-4
Microwave data of ref. [12]
0.10226136(7) 2.405(7) x lo6 1 -
a) Uncertainties represent 20.
375
Volume 127, number 4
CHEMICAL PHYSICS LETTERS
though the vibrationally averaged extension of the HF bond would tend to increase the Ar-F internuclear separation. From estimates of the van der Waals well depths [ 121, vibrationally excited Ar-HF has roughly 34 times enough energy to predissociate. An upper limit to the lifetime can be obtained from the sub-Doppler linewidths of Ar-HF in the slit expansion. The observed fwhm linewidths are approximately 50 MHz, essentially limited by the *25 MHz dye laser frequency jitter measured with an optical spectrum analyzer. (The Ar+ laser jitter is s *5 MHz and does not contribute significantly to the IR linewidth.) This translates into a minimum excited state lifetime of 3 ns, or more than 3 X 10s vibrational periods. Work is presently under way to lock the dye laser to a scanning Fabry-Perot cavity in order to refine this linewidth estimate. We have also observed the van der Waals bending mode, in the v1 + 29 combination band of Ar-HF, 71 cm-l higher than the v1 fundamental. Since this is a ll f Z transition, the (VI t r9) state is split into two opposite parity components, which form the upper levels for the Q and P/R branches, respectively. The I doubling in this state is measured to be negative. The B constant for the v1 t v2 state is significantly lower (-3%) than for the ground state, and indicates a weakening of the van der Waals bond upon excitation of the bend, in contrast to what is observed for excitation of the v1 mode. These data provide an interesting comparison with recent observations of the v2 fundamental [13,14] and v1 t v2 combination band [8] in Ar-HCl. A full analysis of both the v1 and v1 + v2 Ar-HF spectra will be presented in a later publication. This research was supported by National Science Foundation grant PHY82-00805 to the University
376
20 June 1986
of Colorado, as well as grants from the Petroleum Research Fund, Research Corporation, and the Henry and Camille Dreyfus Foundation. CML wishes to acknowledge the Exxon Education Foundation for a graduate fellowship.
References [l] R.J. Leroy, in: Resonances in electron-molecule scattering van der Waals complexes and reactive chemical dynamics, ed. D.G. TruhIar, ACS Symp. Ser. (Am. Chem. Sot. Washington, 1984). [2] J.A. Beswick and J. Jortner, Advan. Chem. Phys. 47 -(1981) 363. [ 31 D.A. Dixon and D.R. Herschbach, Ber. Bunsenges. Physik. Chem. 81 (1977) 145. [4] S.-I Chu, in : Resonances in electron-molecule scattering van der Waals complexes and reactive chemical dynamics, ed. D.G. Truhlar, ACS Symp. Ser. (Am. Chem. Sot., Washington, 1984). [S] J.M.Hutson,CJ.AshtonandRJ.LeRoy,J.Phys. Chem. 87 (1983) 2713. [6] A.R.W. McKelIar and H.L. Welsh, Can. J. Phys. 50 (1972) 1458. [7] A.S. Pine, WJ. Lafferty and BJ. Howard, J. Chem. Phys. 81 (1984) 2939; N. Ohashi and A.S. Pine, J. Chem. Phys. 81 (1984) 73. [ 81 B J. Howard and A.S. Pine, Chem. Phys. Letters 122 (1985) L [9] G.D. Hayman, J. Hodge, BJ. Howard, J.S. Muenter and T.R. Dyke, Chem. Phys. Letters 118 (1985) 12. [lo] J.L. Hall and S.A. Lee, Appl. Phys. Letters 29 (1976) 367. [ll] G. Guelochvlll, Opt. Commun. 19 (1976) 150. [12] SJ. Harris, S.E. Novick and W Klemperer, J. Chem. Phys. 60 (1974) 3208; T.A. Dixon, C.H. Joyner, F.A. Baiocchi and W. Klempere J. Chem. Phys. 74 (1981) 6539. [13] M.D. Marshall, A. Charo, H.O. Leung and W. Kemperer, J. Chem. Phys. 83 (1985) 4924. [ 141 D. Ray, R.L. Robinson, D.-H. Gwo and RJ. SaykaBy, J. Chem. Phys. 84 (1986) 1171.
CHEMICAL PHYSICS LETTERS
ZOJune1986
SUB-DOPPLER INFRARED ABSORPTION SPECTROSCOPY OF Ar-HF I(lO’O) + (oO”O)I IN A LINEAR SUPERSONIC JET Christopher M. LOVETOY, Michael D. SCHUDER and David J. NESBIIT
1
Joint Instituiefor Laboratory Astrophysrcs, Uniuersiiy of Colorado and National Bureau of Standnrdr and Departmenl of Chemistry, Unruersrty of Colorado, Boulder, CO 80309, USA
Received 24 February 1986; in final form 4 April 1986
Ultra-sensitive tunable difference frequency IR absorption spectroscopy in a slit supersonic jet has been used to observe sub-Doppler spectra of Ar-HF in the (lO*O) HF stretch and (11’0) HF stretch plus van der Waals bend modes. Linewidths yield a lower limit of 3 X lo-’ s for the predissociation lifetime in the vibrationally metastable upper state. The sensitivity of these direct absorption methods ( s 2 X lo9 molecules/cm3 per quantum state), in conjunction with the wide tunability of the difference frequency laser (2.2-4.2 pm) permit high-resolution studies of a large class of van der Waals complexes.
Infrared studies of weakly bound van der Waals molecules provide considerable insight into the structure of non-rigid molecules, as well as the dynamics of vibrational predissociation, collisional energy transfer, and intramolecular vibrational redistribution phenomena [l-3]. Interest in small molecular species has been particularly keen due yo the feasibility of accurate theoretical treatments [4,5]. The elegant work of McKellar, Welsh and co-workers on inert gashydrogen complexes took advantage of the infrared transparency of the uncomplexed hydrogen to enable studies in a high pressure, multiple pass cell [6]. pine and co-workers have subsequently utilized Dopplerlimited tunable difference frequency methods to probe hydrogen fluoride and hydrogen chloride dimers (and isotopic variants) in a cooled White cell [7]. Quite recently, these methods have been used to observe inert gas-hydrogen chloride complexes in a 127 K, 72 m optical path length [ 81. Low temperature, supersonic molecular beam experiments offer the possibility of dramatically reducing spectral congestion (both from monomer and van der Waals species) as well as generating a wide variety of weakly bound species not accessible in sufficient concentrations under equilibrium conditions [9]. Such beam studies, however, require exceptional sensitivity ’ Quantum Phystcs Dtvision, National Bureau of Standards.
374
to compensate for the five orders of magnitude loss in path length between a White cell and a conventional pinhole supersonic expansion. In this communication we report the first high resolution, near infrared absorption spectrum of Ar-HF, using tunable difference frequency laser absorption spectroscopy in a supersonic slit expansion. IR absorption by Ar-HF is monitored by double passing a tunable difference frequency laser across a pulsed slit expansion of our own design based on a solenoid activated plunger. The gas pulse has rise and fall times s 100 ps. The slit dimensions are 0.5” by 0.005”; rotational temperatures obtained for ArHF from a 400 Torr (1 Torr = 130 Pa) reservoir (1% HFfAr) are typically 5 10 K at 1.5 cm from the nozzle (see fig. 1). The net 1” path length represents two orders of magnitude enhancement over conventional pinhole expansions. The velocity distribution along the slit dimension and petpendicuZar to the flow is effectively quenched by collisions with adjacent molecules; residual Doppler broadening is reduced to less than the IR laser frequency jitter (k2.5 MHz). The net result is a naturally collimated jet for sub-Doppler spectroscopy without loss of number density X path length. Details of the slit pulsed valve, as well as a full description of the experimental apparatus, will be presented in a later publication. A total of 26 near infrared transitions (P( 12)0 ~9-2614/86/$03.50 0 Elsevier Science Publishers B.V. (Norht-Holland Physics publishing Division)
CHEMICAL PHYSICS LETTERS
Volume 127, number 4
2OJune1986
Ar-HF 10’0) + (00’0) T=B’K
Ar-HF(
IODINE
ETALON 10
lO*O+OO*O)
REFERENCE
FRINGES
-267GHz-
---‘^ 3151.”
WAVENUMBERS
Fig. 1. Stick plot of the raw absorption data for the ur stretch of Ar-HF. The fluctuations are due to variations in IR laser intensity during the scan.
R(13)), in AI-HF have been observed in the vl high frequency stretching mode. A stick plot of the raw absorption data is shown in fig. 1. Fluctuations are due to variations in the IR laser intensity over the duration of the scan. IR transition frequencies are obtained to +O.OOl cm-l uncertainty by measuring both ArI+and dye laser frequencies with a traveling corner cube interferometer [lo] referenced to a polarization stabilized HeNe laser. Absolute frequency calibration is obtained from nearby HF monomer lines [ 1 I]. Typical signal-to-noise in our data is demonstrated in fig. 2 for four pulses signal averaged with a 500 ps boxcar gate (20 Hz.repetition rate, 2.5 min scan time). The ground vibrational state of Ar-HF is known to have a linear equilibrium geometry, with large amplitude zero-point motion in the bending coordinate [ 121. The v1 fundamental spectrum, therefore, will exhibit a simple P and R branch structure as is observed. The null gap between R(0) and P( 1) permits unambiguous J assignment of the lines. Transition frequencies have been fitted to a third-degree polynomial expansion in J(J t 1). The constants obtained are reported in table 1 along with their statistical uncertainties; agreement between the microwave [ 121 and the present near IR data for the ground state constants is seen to be excellent. The rotationless transition frequency in Ar-HF is approximately 10 cm-l red-shifted from the corresponding free HF transition. This can be interpreted
3952600
3952 500
3952 400
I 3952 300
I 3952 2ocl
IR FREQUENCY (cm-‘)
Fig. 2. Sample data scan through the R(3) + R(1) lines of Ar-HF. The upper trace is the absorbance signal averaged over four pulses of the valve with a 500 PCS boxcar gate. Peak absorption is estimated to be 0.2%; noise limit corresponds to roughly lO+ absorbance. The lower two traces are iodine fluorescence and etalon frequency markers for wavelength calibration purposes. The Ar-HF sub-Doppler linewidths are approximately 50 MHz fwhm. limited primarily by frequency jitter (*25 MHz)in the dye laser. Supersonic jet rotational temperatures of 5 10 K are readily achieved in this slit design with stagnation pressures of 400 Torr.
as a weakening of the I-IF bond upon complexation, or equivalently described as due to a greater attraction of Ar for vibrationally excited HF. Additional support for this latter perspective is evidenced in the positive AB for Ar-HF in the u1 = 1 state, i.e. the van der Waals bond contracts slightly upon excitation, even Table 1 Ar-HF rotational constantsa) I(10’0) + (OO”O)I This work (cm-‘) “0
Boo”o
h”o &o”o Dlooo 4lOOO
HlOOO Blo”o - Bw”o
-
3951.770(3) 0.102246(15) 2.34(9) x 106 0.102600(15) 2.04(8) x 1O-e 2.5(16) x lo-to 3.2(14) x lo-lo 3.9(9) x 10-4
Microwave data of ref. [12]
0.10226136(7) 2.405(7) x lo6 1 -
a) Uncertainties represent 20.
375
Volume 127, number 4
CHEMICAL PHYSICS LETTERS
though the vibrationally averaged extension of the HF bond would tend to increase the Ar-F internuclear separation. From estimates of the van der Waals well depths [ 121, vibrationally excited Ar-HF has roughly 34 times enough energy to predissociate. An upper limit to the lifetime can be obtained from the sub-Doppler linewidths of Ar-HF in the slit expansion. The observed fwhm linewidths are approximately 50 MHz, essentially limited by the *25 MHz dye laser frequency jitter measured with an optical spectrum analyzer. (The Ar+ laser jitter is s *5 MHz and does not contribute significantly to the IR linewidth.) This translates into a minimum excited state lifetime of 3 ns, or more than 3 X 10s vibrational periods. Work is presently under way to lock the dye laser to a scanning Fabry-Perot cavity in order to refine this linewidth estimate. We have also observed the van der Waals bending mode, in the v1 + 29 combination band of Ar-HF, 71 cm-l higher than the v1 fundamental. Since this is a ll f Z transition, the (VI t r9) state is split into two opposite parity components, which form the upper levels for the Q and P/R branches, respectively. The I doubling in this state is measured to be negative. The B constant for the v1 t v2 state is significantly lower (-3%) than for the ground state, and indicates a weakening of the van der Waals bond upon excitation of the bend, in contrast to what is observed for excitation of the v1 mode. These data provide an interesting comparison with recent observations of the v2 fundamental [13,14] and v1 t v2 combination band [8] in Ar-HCl. A full analysis of both the v1 and v1 + v2 Ar-HF spectra will be presented in a later publication. This research was supported by National Science Foundation grant PHY82-00805 to the University
376
20 June 1986
of Colorado, as well as grants from the Petroleum Research Fund, Research Corporation, and the Henry and Camille Dreyfus Foundation. CML wishes to acknowledge the Exxon Education Foundation for a graduate fellowship.
References [l] R.J. Leroy, in: Resonances in electron-molecule scattering van der Waals complexes and reactive chemical dynamics, ed. D.G. TruhIar, ACS Symp. Ser. (Am. Chem. Sot. Washington, 1984). [2] J.A. Beswick and J. Jortner, Advan. Chem. Phys. 47 -(1981) 363. [ 31 D.A. Dixon and D.R. Herschbach, Ber. Bunsenges. Physik. Chem. 81 (1977) 145. [4] S.-I Chu, in : Resonances in electron-molecule scattering van der Waals complexes and reactive chemical dynamics, ed. D.G. Truhlar, ACS Symp. Ser. (Am. Chem. Sot., Washington, 1984). [S] J.M.Hutson,CJ.AshtonandRJ.LeRoy,J.Phys. Chem. 87 (1983) 2713. [6] A.R.W. McKelIar and H.L. Welsh, Can. J. Phys. 50 (1972) 1458. [7] A.S. Pine, WJ. Lafferty and BJ. Howard, J. Chem. Phys. 81 (1984) 2939; N. Ohashi and A.S. Pine, J. Chem. Phys. 81 (1984) 73. [ 81 B J. Howard and A.S. Pine, Chem. Phys. Letters 122 (1985) L [9] G.D. Hayman, J. Hodge, BJ. Howard, J.S. Muenter and T.R. Dyke, Chem. Phys. Letters 118 (1985) 12. [lo] J.L. Hall and S.A. Lee, Appl. Phys. Letters 29 (1976) 367. [ll] G. Guelochvlll, Opt. Commun. 19 (1976) 150. [12] SJ. Harris, S.E. Novick and W Klemperer, J. Chem. Phys. 60 (1974) 3208; T.A. Dixon, C.H. Joyner, F.A. Baiocchi and W. Klempere J. Chem. Phys. 74 (1981) 6539. [13] M.D. Marshall, A. Charo, H.O. Leung and W. Kemperer, J. Chem. Phys. 83 (1985) 4924. [ 141 D. Ray, R.L. Robinson, D.-H. Gwo and RJ. SaykaBy, J. Chem. Phys. 84 (1986) 1171.