- Email: [email protected]
Rotational-state-resolved collisional attenuation of hexapole focused hydroxyl radical beams by gas-phase target molecules
28 May 1999
Chemical Physics Letters 305 Ž1999. 348–352
Rotational-state-resolved collisional attenuation of hexapole focused hydroxyl radical beams by gas-phase target molecules Toby D. Hain, Lutz Baars-Hibbe, Thomas J. Curtiss
)
Department of Chemistry, UniÕersity of Utah, Salt Lake City, UT 84112-0850, USA Received 2 October 1998; in final form 22 March 1999
Abstract Hydroxyl radicals from a supersonic discharge source were rotationally state-selected by a hexapole. Two < J V M : states were resolved, the < 32 32 32 : and < 32 32 12 : states. Scattering gases ŽHe, Ar, H 2 O, CO 2 NH 3 , and CH 3 F. were admitted to the hexapole chamber attenuating the hydroxyl radical beam. Analysis of the attenuation as a function of gas pressure yielded the relative scattering cross-sections. Simulations of the focusing spectra suggest that elastic scattering rather than M changing collisions dominate the scattering. No differences in the scattering cross-sections for the < 32 32 32 : and < 32 32 12 : states were observable. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction An electrostatic hexapole will selectively focus a molecular beam of polar molecules according to the Stark behavior of individual rotational states populated in the beam w1,2x. In favorable cases single < J K M : rotational states of symmetric-top molecules have been isolated w3–5x. Recently, Hain et al. have demonstrated a similar capability in isolating rotational states of the hydroxyl radical, specifically, the < JV M : s < 32 32 32 : and < 32 32 12 : states w6x. The utility of such beams stems from the highly anisotropic distribution of lab-frame orientations for molecules in selected states w7,8x. Consequently, hexapoleselected beams have been exploited extensively to probe stereo-selective effects in chemical dynamics w9–11x. ) Corresponding author. Fax: q1 801 581 8433; e-mail: [email protected]
Recently, Harland and coworkers reported a relatively simple experiment that yielded the inelastic Ž M changing. cross-sections for hexapole stateselected CH 3 Cl scattered by the rare gases ŽRg. and N2 w12x. Experimentally they measured the focusing spectra of CH 3 Cl as a function of the pressure of target atoms admitted to their hexapole vacuum chamber. A Beer’s law analysis of the resulting attenuated focusing spectra generated the desired cross-sections for selected CH 3 Cl rotational states. In this Letter, we report our initial results from similar experiments involving hexapole-selected OD radicals. We show that the cross-section for Ar scattering OD from the < 32 32 32 : state is the same as that for the < 23 23 21 : state within our detection limits. Furthermore the attenuation in the focused beam signal was accompanied by substantial peak broadening, an observation that convinces us that the dominant scattering mechanism was elastic scattering rather than M changing collisions.
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 3 9 4 - 2
T.D. Hain et al.r Chemical Physics Letters 305 (1999) 348–352
349
2. Experimental The hexapole focusing apparatus used in these experiments has been described in detail elsewhere w13x. A 1:15 ArrHe mixture was bubbled through D 2 O at room temperature and expanded from a corona discharge source w14x to generate a beam of OD radicals characterized by a stream velocity of 1450 m sy1 and a translational temperature of 4 K. Velocities were measured using standard time-offlight ŽTOF. techniques. Rotational states of OD radicals were selectively focused into the ionizer of an electron impact quadrupole mass spectrometer by an electrostatic hexapole. The chamber containing the hexapole had an entrance collimator 3.4 mm in diameter and exit collimator 2.5 mm in diameter and was 1950 mm long Žsee Ref. w13x for more details.. Scattering gases were admitted to the hexapole chamber through a leak valve and pressures were measured with a Bayard–Alpert style ion gauge. Pressure readings were corrected for the gauge sensitivity relative to N2 .
3. Results Fig. 1 shows a series of OD focusing spectra measured as a function of Ar pressure in the hexapole
Fig. 1. OD focusing spectra measured as a function of the partial pressure of Ar in the hexapole chamber Žright column..
Fig. 2. Beer’s law plots of the OD focused beam intensities shown in Fig. 1 at V0 s 7, 18 and 24 kV corresponding to the < J V M ) s < 32 32 32 :, < 32 32 12 :, and < 32 32 32 : states, respectively. Different slopes incorrectly suggest different scattering cross-sections Žsee text..
chamber. The three features evident in each spectrum correspond to the half-wave trajectory of the < 32 32 32 : state at V0 s 7 kV, the half-wave trajectory of the < 32 32 12 : state at V0 s 18 kV, and the full-wave trajectory of the < 23 23 23 : state at V0 s 24 kV. A Beer’s law plot of the attenuated signal at a given value V0 versus pressure yields an estimate of the scattering cross-section for rotational states populating the beam. Such plots are shown in Fig. 2 for intensities attenuated at V0 s 7, 18 and 24 kV. The difference seen in the slope of these plots suggested to us that the cross-sections might be different for rotational states populating the beam at these voltages. While the difference seen between the < 32 32 32 : Ž7 kV. and < 23 23 21 : Ž18 kV. might be attributed to steric effects, it was hard to rationalize the dramatic difference in cross-sections observed for the < 32 32 32 : at V0 s 7 kV and the same state at V0 s 24 kV. To better understand this behavior we performed careful classical trajectory calculations to simulate the experimentally measured focusing spectra in Fig. 1 w13x. These simulations revealed that an appreciable fraction Ž24%. of the OD radicals focused by the hexapole were populating the < 52 32 M : states in addition to the < 32 23 M : states. Fig. 3a shows a
350
T.D. Hain et al.r Chemical Physics Letters 305 (1999) 348–352
1 = 10y5 Pa. of gas in the hexapole chamber. The < 52 32 M : state contributions to the beam are indicated with the fine dotted line and the < 23 23 M : with the fine solid line. In previous simulations we have assumed that the abundance of the < 32 32 32 : and < 32 32 12 : states in the beam were the same w13x. In this study we simulated the focusing spectra of the < 32 32 32 : and < 32 32 12 : states independently and multiplied each by an adjustable scaling parameter that optimized a least-squares fit to the measured spectrum. We found that a 1:1 ratio of populations in the < 32 32 32 : to < 23 23 21 : produced the optimum fit for the focusing spectrum taken with only a background pressure in the hexapole chamber. This validates the assumptions we have made previously. By selectively scaling the simulated focusing spectrum for each rotational state contributing to focused beam and optimizing the fit between the resulting simulated spectrum and the experimental we believed the scaling factors might give us a good measure of the scattering cross-section for each state. In practice, however, we were unable achieve a satisfactory fit to the experimental spectra using this procedure. The problem was that the shapes of spectra were changing for individual states in addition to
Fig. 3. Panel Ža. shows a comparison between a simulated focusing spectrum Žheavy solid line. and the experimental OD focusing spectrum Žpoints. taken with only a background pressure in the hexapole chamber Ž PAr s 0 Pa.. The light solid line shows the contributions from the < 32 32 M : states and the light dotted line the contributions from the < 52 32 M : states. Panel Žb. demonstrates our inability to fit an attenuated OD spectrum Ž PAr s 7.8=10y4 Pa. under the same beam conditions ŽTS s 4 K. due to peak broadening. Panel Žc. shows a comparison of the same attenuated spectrum with a simulation obtained by increasing the width of the velocity distribution ŽTS s8 K..
simulated focusing spectrum Žheavy solid line. compared with the experimental data Ždots. for an OD focusing spectrum with only a low background Ž P s
Fig. 4. Beer’s law plots of the attenuation in the peak intensity for the first appearance of the < 32 32 32 : rotational state in the OD focusing spectrum for a variety of target molecules.
T.D. Hain et al.r Chemical Physics Letters 305 (1999) 348–352
their intensities. We also found it impossible to simultaneously fit the half-wave and full-wave components of the < 32 32 32 : state. This is illustrated in Fig. 3b where we have attempted to fit the attenuated spectrum corresponding to an Ar pressure of 7.8 = 10y4 Pa by the procedure outlined above. It is clear from the comparison that the experimental spectrum is less well resolved than the simulated spectrum. This is a consequence of elastic collisions broadening the velocity distribution of molecules still being successfully transmitted to the detector. This is particularly evident for the narrow < 32 32 32 : state focused at V0 s 7 kV. Herein lies the origin of the apparent differences in cross-sections observed in Fig. 2. Because the widths of focusing features naturally increase with increasing hexapole voltage, the velocity broadening effects of elastic collisions will always be most dramatic for the narrow low voltage features. The cross-sections determined from monitoring the peak height at particular voltages will simply scale inversely with hexapole voltage as is seen in Fig. 2. Fig. 3c validates this assertion. In this panel, we have increased the translational temperature in the simulation from that ascertained from TOF measurements of the direct, unattenuated beam, TS s 4–8 K. The simulations in Fig. 3c capture the peak broadening caused by collisions at large impact parameters. Indicated on each of the panels of Fig. 3 are the translational temperatures and the ratios of the populations of the < 32 32 32 : state to the < 32 32 12 : state. The agreement between the spectra in Fig. 3c clearly demonstrates that the apparent differences in cross-sections in Fig. 2 are an artifact of relying on peak heights to ascertain rotational state populations. Consequently, we believe there is no evidence in our data for preferential scattering out of any of the states resolved in our focusing spectra. We performed similar scattering experiments for a variety of scattering targets. Beer’s law plots are shown in Fig. 4. The cross-sections were measured by monitoring the attenuation in the first peak of the < 32 32 32 : state. Table 1 shows a list of the measured cross-sections relative to that for Ar along with the dipole moments and polarizabilities of all the target gases. The absolute cross-sections are quite large ˚ 2 . even for elastic scattering. There is Ž sAr ; 500 A no doubt that there is considerable uncertainty in the absolute cross-sections, however, the relative cross-
351
Table 1 Relative cross-sections, polarizabilities, and electric dipoles for target gas M M
sM r sAr
˚ 3 .a a Ž= A
m ŽD. a
He Ar H 2O CO 2 NH 3 CH 3 F
0.4 1.0 2.2 2.7 4.9 5.7
0.205 1.64 1.45 2.91 2.40 2.97
1.85 1.47 1.86
a
Ref. w15x.
sections should be accurate to within 10% or so. In addition the cross-sections qualitatively scale with polarizability and dipole moment as would be expected if long-range electrostatic interactions were responsible for the scattering interaction.
Acknowledgements This research was supported by a grant for the NSF NYI Program ŽCHE9457382.. We are grateful to Dr. David Blunt for suggesting this project.
References w1x D.H. Parker, R.B. Bernstein, Ann. Rev. Phys. Chem. 40 Ž1989. 561. w2x J. Reuss, in: G. Scoles ŽEd.., Atomic and Molecular Beam Methods, Oxford University Press, New York, 1988. w3x S.R. Gandhi, T.J. Curtiss, Q.-X. Xu, S.E. Choi, R.B. Bernstein, Chem. Phys. Lett. 132 Ž1986. 6. w4x D.H. Parker, H. Jalink, S. Stolte, J. Phys. Chem. 91 Ž1987. 5427. w5x D.-C. Che, T. Ogawa, H. Ohoyama, T. Kasai, K. Kuwata, Bull. Chem. Soc. Jpn 68 Ž1995. 771. w6x T.D. Hain, M.A. Weibel, K.M. Backstrand, T.J. Curtiss, J. Phys. Chem. A 101 Ž1997. 7674. w7x S.E. Choi, R.B. Bernstein, J. Chem. Phys. 85 Ž1986. 150. w8x T.D. Hain, T.J. Curtiss, J. Phys. Chem. A 102 Ž1998. 9696. w9x Dynamical Stereochemistry, special issue of J. Phys. Chem. 91 Ž21. Ž1987.. w10x Orientation and Polarization Effects in Reactive Collisions,
352
T.D. Hain et al.r Chemical Physics Letters 305 (1999) 348–352
special issue of J. Chem. Soc., Faraday Trans. 2 85 Ž8. Ž1989.. w11x Stereodynamics of Chemical Reactions, special issue of J. Phys. Chem. A 101 Ž41. Ž1997.. w12x D.A. Blunt, S.A. Harris, W. Hu, P.W. Harland, J. Phys. Chem. A 102 Ž1998. 1482.
w13x T.D. Hain, M.A. Weibel, K.M. Backstrand, T.J. Curtiss, J. Phys. Chem. A 101 Ž1997. 7674. w14x P.C. Engelking, Rev. Sci. Instrum. 57 Ž1986. 2274. w15x D.R. Lide ŽEd.., CRC Handbook of Chemistry and Physics, 74th edn., CRC Press, Boca Raton, FL, 1993.
Chemical Physics Letters 305 Ž1999. 348–352
Rotational-state-resolved collisional attenuation of hexapole focused hydroxyl radical beams by gas-phase target molecules Toby D. Hain, Lutz Baars-Hibbe, Thomas J. Curtiss
)
Department of Chemistry, UniÕersity of Utah, Salt Lake City, UT 84112-0850, USA Received 2 October 1998; in final form 22 March 1999
Abstract Hydroxyl radicals from a supersonic discharge source were rotationally state-selected by a hexapole. Two < J V M : states were resolved, the < 32 32 32 : and < 32 32 12 : states. Scattering gases ŽHe, Ar, H 2 O, CO 2 NH 3 , and CH 3 F. were admitted to the hexapole chamber attenuating the hydroxyl radical beam. Analysis of the attenuation as a function of gas pressure yielded the relative scattering cross-sections. Simulations of the focusing spectra suggest that elastic scattering rather than M changing collisions dominate the scattering. No differences in the scattering cross-sections for the < 32 32 32 : and < 32 32 12 : states were observable. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction An electrostatic hexapole will selectively focus a molecular beam of polar molecules according to the Stark behavior of individual rotational states populated in the beam w1,2x. In favorable cases single < J K M : rotational states of symmetric-top molecules have been isolated w3–5x. Recently, Hain et al. have demonstrated a similar capability in isolating rotational states of the hydroxyl radical, specifically, the < JV M : s < 32 32 32 : and < 32 32 12 : states w6x. The utility of such beams stems from the highly anisotropic distribution of lab-frame orientations for molecules in selected states w7,8x. Consequently, hexapoleselected beams have been exploited extensively to probe stereo-selective effects in chemical dynamics w9–11x. ) Corresponding author. Fax: q1 801 581 8433; e-mail: [email protected]
Recently, Harland and coworkers reported a relatively simple experiment that yielded the inelastic Ž M changing. cross-sections for hexapole stateselected CH 3 Cl scattered by the rare gases ŽRg. and N2 w12x. Experimentally they measured the focusing spectra of CH 3 Cl as a function of the pressure of target atoms admitted to their hexapole vacuum chamber. A Beer’s law analysis of the resulting attenuated focusing spectra generated the desired cross-sections for selected CH 3 Cl rotational states. In this Letter, we report our initial results from similar experiments involving hexapole-selected OD radicals. We show that the cross-section for Ar scattering OD from the < 32 32 32 : state is the same as that for the < 23 23 21 : state within our detection limits. Furthermore the attenuation in the focused beam signal was accompanied by substantial peak broadening, an observation that convinces us that the dominant scattering mechanism was elastic scattering rather than M changing collisions.
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 3 9 4 - 2
T.D. Hain et al.r Chemical Physics Letters 305 (1999) 348–352
349
2. Experimental The hexapole focusing apparatus used in these experiments has been described in detail elsewhere w13x. A 1:15 ArrHe mixture was bubbled through D 2 O at room temperature and expanded from a corona discharge source w14x to generate a beam of OD radicals characterized by a stream velocity of 1450 m sy1 and a translational temperature of 4 K. Velocities were measured using standard time-offlight ŽTOF. techniques. Rotational states of OD radicals were selectively focused into the ionizer of an electron impact quadrupole mass spectrometer by an electrostatic hexapole. The chamber containing the hexapole had an entrance collimator 3.4 mm in diameter and exit collimator 2.5 mm in diameter and was 1950 mm long Žsee Ref. w13x for more details.. Scattering gases were admitted to the hexapole chamber through a leak valve and pressures were measured with a Bayard–Alpert style ion gauge. Pressure readings were corrected for the gauge sensitivity relative to N2 .
3. Results Fig. 1 shows a series of OD focusing spectra measured as a function of Ar pressure in the hexapole
Fig. 1. OD focusing spectra measured as a function of the partial pressure of Ar in the hexapole chamber Žright column..
Fig. 2. Beer’s law plots of the OD focused beam intensities shown in Fig. 1 at V0 s 7, 18 and 24 kV corresponding to the < J V M ) s < 32 32 32 :, < 32 32 12 :, and < 32 32 32 : states, respectively. Different slopes incorrectly suggest different scattering cross-sections Žsee text..
chamber. The three features evident in each spectrum correspond to the half-wave trajectory of the < 32 32 32 : state at V0 s 7 kV, the half-wave trajectory of the < 32 32 12 : state at V0 s 18 kV, and the full-wave trajectory of the < 23 23 23 : state at V0 s 24 kV. A Beer’s law plot of the attenuated signal at a given value V0 versus pressure yields an estimate of the scattering cross-section for rotational states populating the beam. Such plots are shown in Fig. 2 for intensities attenuated at V0 s 7, 18 and 24 kV. The difference seen in the slope of these plots suggested to us that the cross-sections might be different for rotational states populating the beam at these voltages. While the difference seen between the < 32 32 32 : Ž7 kV. and < 23 23 21 : Ž18 kV. might be attributed to steric effects, it was hard to rationalize the dramatic difference in cross-sections observed for the < 32 32 32 : at V0 s 7 kV and the same state at V0 s 24 kV. To better understand this behavior we performed careful classical trajectory calculations to simulate the experimentally measured focusing spectra in Fig. 1 w13x. These simulations revealed that an appreciable fraction Ž24%. of the OD radicals focused by the hexapole were populating the < 52 32 M : states in addition to the < 32 23 M : states. Fig. 3a shows a
350
T.D. Hain et al.r Chemical Physics Letters 305 (1999) 348–352
1 = 10y5 Pa. of gas in the hexapole chamber. The < 52 32 M : state contributions to the beam are indicated with the fine dotted line and the < 23 23 M : with the fine solid line. In previous simulations we have assumed that the abundance of the < 32 32 32 : and < 32 32 12 : states in the beam were the same w13x. In this study we simulated the focusing spectra of the < 32 32 32 : and < 32 32 12 : states independently and multiplied each by an adjustable scaling parameter that optimized a least-squares fit to the measured spectrum. We found that a 1:1 ratio of populations in the < 32 32 32 : to < 23 23 21 : produced the optimum fit for the focusing spectrum taken with only a background pressure in the hexapole chamber. This validates the assumptions we have made previously. By selectively scaling the simulated focusing spectrum for each rotational state contributing to focused beam and optimizing the fit between the resulting simulated spectrum and the experimental we believed the scaling factors might give us a good measure of the scattering cross-section for each state. In practice, however, we were unable achieve a satisfactory fit to the experimental spectra using this procedure. The problem was that the shapes of spectra were changing for individual states in addition to
Fig. 3. Panel Ža. shows a comparison between a simulated focusing spectrum Žheavy solid line. and the experimental OD focusing spectrum Žpoints. taken with only a background pressure in the hexapole chamber Ž PAr s 0 Pa.. The light solid line shows the contributions from the < 32 32 M : states and the light dotted line the contributions from the < 52 32 M : states. Panel Žb. demonstrates our inability to fit an attenuated OD spectrum Ž PAr s 7.8=10y4 Pa. under the same beam conditions ŽTS s 4 K. due to peak broadening. Panel Žc. shows a comparison of the same attenuated spectrum with a simulation obtained by increasing the width of the velocity distribution ŽTS s8 K..
simulated focusing spectrum Žheavy solid line. compared with the experimental data Ždots. for an OD focusing spectrum with only a low background Ž P s
Fig. 4. Beer’s law plots of the attenuation in the peak intensity for the first appearance of the < 32 32 32 : rotational state in the OD focusing spectrum for a variety of target molecules.
T.D. Hain et al.r Chemical Physics Letters 305 (1999) 348–352
their intensities. We also found it impossible to simultaneously fit the half-wave and full-wave components of the < 32 32 32 : state. This is illustrated in Fig. 3b where we have attempted to fit the attenuated spectrum corresponding to an Ar pressure of 7.8 = 10y4 Pa by the procedure outlined above. It is clear from the comparison that the experimental spectrum is less well resolved than the simulated spectrum. This is a consequence of elastic collisions broadening the velocity distribution of molecules still being successfully transmitted to the detector. This is particularly evident for the narrow < 32 32 32 : state focused at V0 s 7 kV. Herein lies the origin of the apparent differences in cross-sections observed in Fig. 2. Because the widths of focusing features naturally increase with increasing hexapole voltage, the velocity broadening effects of elastic collisions will always be most dramatic for the narrow low voltage features. The cross-sections determined from monitoring the peak height at particular voltages will simply scale inversely with hexapole voltage as is seen in Fig. 2. Fig. 3c validates this assertion. In this panel, we have increased the translational temperature in the simulation from that ascertained from TOF measurements of the direct, unattenuated beam, TS s 4–8 K. The simulations in Fig. 3c capture the peak broadening caused by collisions at large impact parameters. Indicated on each of the panels of Fig. 3 are the translational temperatures and the ratios of the populations of the < 32 32 32 : state to the < 32 32 12 : state. The agreement between the spectra in Fig. 3c clearly demonstrates that the apparent differences in cross-sections in Fig. 2 are an artifact of relying on peak heights to ascertain rotational state populations. Consequently, we believe there is no evidence in our data for preferential scattering out of any of the states resolved in our focusing spectra. We performed similar scattering experiments for a variety of scattering targets. Beer’s law plots are shown in Fig. 4. The cross-sections were measured by monitoring the attenuation in the first peak of the < 32 32 32 : state. Table 1 shows a list of the measured cross-sections relative to that for Ar along with the dipole moments and polarizabilities of all the target gases. The absolute cross-sections are quite large ˚ 2 . even for elastic scattering. There is Ž sAr ; 500 A no doubt that there is considerable uncertainty in the absolute cross-sections, however, the relative cross-
351
Table 1 Relative cross-sections, polarizabilities, and electric dipoles for target gas M M
sM r sAr
˚ 3 .a a Ž= A
m ŽD. a
He Ar H 2O CO 2 NH 3 CH 3 F
0.4 1.0 2.2 2.7 4.9 5.7
0.205 1.64 1.45 2.91 2.40 2.97
1.85 1.47 1.86
a
Ref. w15x.
sections should be accurate to within 10% or so. In addition the cross-sections qualitatively scale with polarizability and dipole moment as would be expected if long-range electrostatic interactions were responsible for the scattering interaction.
Acknowledgements This research was supported by a grant for the NSF NYI Program ŽCHE9457382.. We are grateful to Dr. David Blunt for suggesting this project.
References w1x D.H. Parker, R.B. Bernstein, Ann. Rev. Phys. Chem. 40 Ž1989. 561. w2x J. Reuss, in: G. Scoles ŽEd.., Atomic and Molecular Beam Methods, Oxford University Press, New York, 1988. w3x S.R. Gandhi, T.J. Curtiss, Q.-X. Xu, S.E. Choi, R.B. Bernstein, Chem. Phys. Lett. 132 Ž1986. 6. w4x D.H. Parker, H. Jalink, S. Stolte, J. Phys. Chem. 91 Ž1987. 5427. w5x D.-C. Che, T. Ogawa, H. Ohoyama, T. Kasai, K. Kuwata, Bull. Chem. Soc. Jpn 68 Ž1995. 771. w6x T.D. Hain, M.A. Weibel, K.M. Backstrand, T.J. Curtiss, J. Phys. Chem. A 101 Ž1997. 7674. w7x S.E. Choi, R.B. Bernstein, J. Chem. Phys. 85 Ž1986. 150. w8x T.D. Hain, T.J. Curtiss, J. Phys. Chem. A 102 Ž1998. 9696. w9x Dynamical Stereochemistry, special issue of J. Phys. Chem. 91 Ž21. Ž1987.. w10x Orientation and Polarization Effects in Reactive Collisions,
352
T.D. Hain et al.r Chemical Physics Letters 305 (1999) 348–352
special issue of J. Chem. Soc., Faraday Trans. 2 85 Ž8. Ž1989.. w11x Stereodynamics of Chemical Reactions, special issue of J. Phys. Chem. A 101 Ž41. Ž1997.. w12x D.A. Blunt, S.A. Harris, W. Hu, P.W. Harland, J. Phys. Chem. A 102 Ž1998. 1482.
w13x T.D. Hain, M.A. Weibel, K.M. Backstrand, T.J. Curtiss, J. Phys. Chem. A 101 Ž1997. 7674. w14x P.C. Engelking, Rev. Sci. Instrum. 57 Ž1986. 2274. w15x D.R. Lide ŽEd.., CRC Handbook of Chemistry and Physics, 74th edn., CRC Press, Boca Raton, FL, 1993.