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Vibrational effects in proton and charge transfer in the H+2 + Ar system
Volume 82, number 3
VIBRATIONAL
CHEbIKXL
EFFECTS
PHYSICS LETTERS
15 September 1981
IN PROTON AND CHARGE TRANSFER
F.A HOULE *, S.L. ANDERSON
*, D. GERLICH
+> T. TURNER
IN THE H; + Ar SYSTEM
** and Y.T. LEE
Materinls aud Molecular Research Division, Lawreme Berkeley Laboratoty and Department of Chemistry, Utmersity of California, Berkeley, California 94 720, USA Received 27 July 1981
Reaction and charge transfer of Hb c Ar to give ArH+ and Ar+ have been mvestigated as a function of H; vibrational quantum state and kinetic energy (Ecmm_). usmg photomnizatron and gurded beam ran optics. Resonance effects are important in charge transfer, proton and charge transfer are closely coupled for E, m > 3 eV.
1. Introduction Theoretical studres of charge-transfer and protontransfer processes in the (Ar--Hz)+ system, AH=
H,i-Ar+-&+Ar.
H;+Ar
-0.13,
(1)
+ArH+tH,
AH= -1.61
)
(3
+Ar++H,,
aH=+0.31
_
(3)
+ArH++H,
AH=
-1.30
(4)
indicate that although the reactions are direct_ they involve several potential surfaces which are very close in ener,T [l-3]. Transitions between these surfaces during the course of a collision are found to be very important. especially in reaction (2) where the reagents do not correlate to grcund-state products [4] _ The probabrlity of undergoing such transitions depends strongly on vrbrational motion of H2 or 6 as the system moves through an avoided crossing region. The transitions - or lack of them - will dictate the observable branching ratios between (1) and (2) or (3) and (4).
Experimental
verification
of the vibrational
state
* IBM Postdoctoral Fellow Present address: IBM Research Laboratory, San Jose, California 95193, USA. * NSF Predoctoral Fellow. * Present address- Fakuktt fur Physik der Universitat, Frieberg i/Br, W. Germany. ** Fannie and John Hertz Foundation Fellow.
392
dependent surface hopping from the measurement of integral and differential cross sections has been limited by the difficulty of producing H2 and HT in known vibrational states, so much of the published work is restricted to electronic state selected Ar+(2P3,7_, 2P,,,) + H3(u = 0) [S-7] . Recently, several studies of reactions (3) and (4) using crossed molecular beams have been carried out [S-lo] _ It was found that proton transfer is direct, but that the dynamics are more complex than a simple stripping process. Arguments were made that high HAr+ rotational excitation allowed formation of product with internal energy well above the dissociation limit. Partial state section of the H$ vibration led to the conclusion that, at fared collision energy, reagent vibration has little or no effect on the reaction. In addition to these crossed molecular beam studies, the cross sections of reactions (3) and (4) have been studied at several collision energies as a function of vibrational quantum states using photoionization to select the II$ vibrational state [ 1 l-131 _ As found iu the beam experiments, at low relative translational energies (
Volume 82, number 3
CHEMICAL PHYSICS LE-MERS
photoionization and the guided ion beams method [ 141 has been used to investigate energy effects on Ar+ and ArH+ formatlon for G(u = 04) and collision energies of l-10 eV.
15 September 1981
$Ar-Kr v=
0
+H2
I
2
3
H;tAr -ArH++H 4
ECM
0
I
2
3
4 Ect.4
70 50-
9,. Experimental The instrumentation used in this work has been described in detail elsewhere [15]. Briefly, it consists of a He discharge lamp (Hopfield continuum), vacuum ultraviolet monochromator, molecular beam photoionization source for g, radiofrequency octopole ion guides operated at 15 MHz, a scattering cell, quadrupole mass spectrometer and Daly detector_ H$ ions are formed within the ion guides by photoionization of a supersonic beam of Hz. Photoionization with wavelengthselected photons permits generation of an ion beam having a well-defined distribution of vibrational states. The ions are then guided through a differentially pumped region, accelerated to the desired collision energy, and passed through a scattering cell. All reactant and product ions are subsequently collected, mass analyzed, and detected. Variation of ionizing wavelength and collision energy allows determination of integral reaction cross sections as a function of vIbrational state and kinetic energy.
3. Results The data reactions (3) and (4) are presented in fig. 1, &I which relative cross sections for u = 04 at several center-of-mass energies are compared_ The details of the data analysis are described elsewhere [ 151_ Briefly, raw experimental data were checked for internal consistency between variable wavelength and variable kinetic energy scans. Vibrational distributions generated at each wavelength were combined with raw cross sections measured to obtain reaction cross sections as a function of vibrational state. The estimated relative error for the proton-transfer cross sections is +5%. For the charge-transfer measurements, the backstreaming of gas from the scattering cell to the photoionization region gives an additional source of error. If the photoionization efficiency curve were constant for Ar over the wavelength range used, the error introduced would be constant. This is not the case [ 161, however, and
30-
r-
I
20NT-5 b IO7532-
Fig. 1. %&rational state dependence of proton- and chargetram&r uoss sections at several kinetic energies 1 V (-), 3V(-•-_),6V(----_)and9V(---)
+ c
our control experiments indicate that appropriate error bars for charge transfer are -tS%, *7% and al 5% for Hz@ = 0, l), H~(u = 2,3) and Hz(u = 4) respectively. Absolute values of cross sections shown in fig_ 2 indicate that at low kinetic energies (< 1 eV) the
eV
2
Hz+ A’r
+Ar
v=o
16.6 V= 164 16.2 160
I
____ IR-_----
4
_____--
3
2py2 _-----~__-_~~---_
2
2b2 _-_=-_-
I--
15s
v=
2P
_-_____
_
_
2p312 I __-~-_-_-__--~___-_
15.6. m
0
____---
______
_---_
Fig. 2. Schematic of energy levels in the H’: + Ar* + Hz systems.
393
Volume 82, number 3
CHEMICAL PHYSICS LETTERS
charge-transfer and proton-transfer cross sections are similar in magnitude, in agreement with previous woik on both H2 t Ar+ and Ar + Hzf_ The most important features to note are (I) the very small charge-transfer cross section for u = 0 compared to u > 0 at all energies, and (2) the strong resemblance between the vibrational dependence of both reactions at kinetic energies >3 eV.
4. Discussion Theoretical treatments of long-range charge transfer [17,1 S] have shown that both energy resonance and favorable Franck-Condon overlaps enhance the process. A diagram of the energy levels appropriate to the H2f + Ar system is shown in fig_ 2. It can be seen that charge transfer from Hi(u > 0) IS near-resonant, while transfer from H$(u = 0) is 0.3 1 eV endothermic. It is only this last state that requires translational to vibrational energy transfer during the collision; indeed the cross sections *are much smaller for this state than for the higher vibrational levels. It can be seen that dependence of the cross sections on translational energy is quite weak despite the existence of energy level mismatches. Peaking of the cross section for u = 2 coincides with both a closely resonant pair of states and the maximum of the Franck-Condon overlap between H2f and H, [ 193 _ These features of the data indicate that near-resonant charge transfer is a predominantly long-range process, in agreement with the molecular beam results. There are significant differences between the results in fig_ 1 and those obtained in photoion-photoelectron coincidence measurements [12,13]. In both of these studies the increase in cross section from u = 0 to u = 1 or 2 is smaller than observed here. At low kinetic energy, the cross section for u = 2 is maximum in our data and those of Tanaka 1133, but the degree of enhancement of u = 1 over u = 0 is larger in the present work, and the fall-off for u = 3 and 4 much smaller. Although our data only exterld up to 10 eV, the rather weak translational energy dependence allows at least a qualitative comparison to be made to the work of Campbell at >20 eV 1123. Their cross sections show amaximumatu=1ratherthanu=2.‘IBisshiftis rather surprising, and not readily explained. Calculations of cross sections at 20 eV do not predict such a shift [12]. 394
15 September 1981
Theoretical treatments of the proton-transfer channel have shown that only Hz f Ar correlates to ArH+. Since H2f(u = 0) has little probability of undergoing charge transfer (a transition to an unreactive surface), it might be expected that direct proton transfer would be more facile for this state than for the higher vibrational states. In fact at ail energies vibration enhances proton transfer_ Furthermore, the strong resemblance between the vibrational dependences for charge and proton transfer at higher energies suggests that above =3 eV, both channels involve passage through a common region of the potential energy surfaces available to the system. Failure to do so may result in largely nonreactive scattering_ Regardless of the details of the collisions, it is not likely that proton transfer only results from simple motion on the Hzf + Ar surface. Comparison of the proton transfer data at 1 eV with those of Tanaka [ 133 indicate that while the observed trends are in agreement, the vibrational enhancement reported here is more dramatic. Poor beam intensities at 0.3 eV preclude measurement of cross sections for comparison to the data of Chupka [ 111, who reported little vibrational effect at that collision energy. In crossed beam experiments [lo], proton transfer from Hz(u < 2) was compared to Hz (all states), and little difference was found. This result is supported by the data in fig. 1, which indicates that the major variation in vibrational effects involve the states u = O-2. The use of vibrationally state-selected ions over a range of kinetic energies has been crucial in revealing new aspects of an often-studied reactive system. Work in progress includes other systems where near-resonant charge transfer is likely to play an important role in the reaction dynamics such as I$ + N2 _
Acknowledgement This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy under Contract Number W-7405-ENG-48.
References [l] P.J. Kuntz and AS. II 68 (1972) 259.
Roach,
J- Chem.
Sot.
Faraday
Trans.
Vohrme 82, number 3
CHEMICAL
[2] S. Chapman and R.K. Preston, J. Chem. Phys. 60 (1974) 650. [3] M. Baer and J.A. Beswick, Phys. Rev. A19 (1979) 1559. [4] B.H. Mahan, Accounts Chem. Res. 8 (197.5) 55. [5] M. Chiang, E.A. GisIason, B H. Mahan, C-W. Tsao and AS. Werner, J. Chem. Phys. 52 (1970) 2698. [6] P M. Hierl, V Pa& and Z. Herman, 3. Chem. Phys. 67 (1977) 2678. [7] K. Tanaka, J. Durup, T Kato and I. Koyano, JXhem. Phys. 73 (1980) 586. [S] R.M BiIotta, F.N. Preuninger and J M. Farrar, Chem. Phys Letters 74 (1980) 95. [9] R.M. BiIotta, F.N. Preuninger and J_hI. Farrar, J. Chem. Phys. 73 (1980) 1637. [lo] R M. Bdotta and J-M. Farrar, J. Chem. Phys. 74 (1981) 1699. [ll] W A. Chupka and M E. Russell, J. Chem. Phys. 49 (1968) 5426
PHYSICS LETI’ERS
15 September 1981
[12] FM. Campbell, R. Browning and C.J. Latimer, J. Phys. B13 (1980) 4257. [ 131 K. Tanaka, T. Karo and I. Koyano, XII JCPZAC, Gatlinburg, Tennessee, July 198 1. [ 141 E. Teloy and D. GerIich, Chem. Phys. 4 (1974) 417. [lS] S.L. Anderson, F-A. HouIe, D. Gerlich and Y-T. Lee, 3. Chem. Phys. (Sept. 1,1981), to be published. [16] R.E. Huffman, Y. Tanaka and J-C. Lan-abee, J. Chem. Phys. 39 (1965) 902. [17] D R Bates and R H.G. Reid, Proc. Roy. Sot. A310 (1969‘) 1.
[18] M-T_ Bowers and T. Su, Advan. Electron. Electron Phys. 34 (1973) 231. [19] J. Berkowrtz and R. Spohr. J. Electron Spectry. 2 (1973) 143.
395
VIBRATIONAL
CHEbIKXL
EFFECTS
PHYSICS LETTERS
15 September 1981
IN PROTON AND CHARGE TRANSFER
F.A HOULE *, S.L. ANDERSON
*, D. GERLICH
+> T. TURNER
IN THE H; + Ar SYSTEM
** and Y.T. LEE
Materinls aud Molecular Research Division, Lawreme Berkeley Laboratoty and Department of Chemistry, Utmersity of California, Berkeley, California 94 720, USA Received 27 July 1981
Reaction and charge transfer of Hb c Ar to give ArH+ and Ar+ have been mvestigated as a function of H; vibrational quantum state and kinetic energy (Ecmm_). usmg photomnizatron and gurded beam ran optics. Resonance effects are important in charge transfer, proton and charge transfer are closely coupled for E, m > 3 eV.
1. Introduction Theoretical studres of charge-transfer and protontransfer processes in the (Ar--Hz)+ system, AH=
H,i-Ar+-&+Ar.
H;+Ar
-0.13,
(1)
+ArH+tH,
AH= -1.61
)
(3
+Ar++H,,
aH=+0.31
_
(3)
+ArH++H,
AH=
-1.30
(4)
indicate that although the reactions are direct_ they involve several potential surfaces which are very close in ener,T [l-3]. Transitions between these surfaces during the course of a collision are found to be very important. especially in reaction (2) where the reagents do not correlate to grcund-state products [4] _ The probabrlity of undergoing such transitions depends strongly on vrbrational motion of H2 or 6 as the system moves through an avoided crossing region. The transitions - or lack of them - will dictate the observable branching ratios between (1) and (2) or (3) and (4).
Experimental
verification
of the vibrational
state
* IBM Postdoctoral Fellow Present address: IBM Research Laboratory, San Jose, California 95193, USA. * NSF Predoctoral Fellow. * Present address- Fakuktt fur Physik der Universitat, Frieberg i/Br, W. Germany. ** Fannie and John Hertz Foundation Fellow.
392
dependent surface hopping from the measurement of integral and differential cross sections has been limited by the difficulty of producing H2 and HT in known vibrational states, so much of the published work is restricted to electronic state selected Ar+(2P3,7_, 2P,,,) + H3(u = 0) [S-7] . Recently, several studies of reactions (3) and (4) using crossed molecular beams have been carried out [S-lo] _ It was found that proton transfer is direct, but that the dynamics are more complex than a simple stripping process. Arguments were made that high HAr+ rotational excitation allowed formation of product with internal energy well above the dissociation limit. Partial state section of the H$ vibration led to the conclusion that, at fared collision energy, reagent vibration has little or no effect on the reaction. In addition to these crossed molecular beam studies, the cross sections of reactions (3) and (4) have been studied at several collision energies as a function of vibrational quantum states using photoionization to select the II$ vibrational state [ 1 l-131 _ As found iu the beam experiments, at low relative translational energies (
Volume 82, number 3
CHEMICAL PHYSICS LE-MERS
photoionization and the guided ion beams method [ 141 has been used to investigate energy effects on Ar+ and ArH+ formatlon for G(u = 04) and collision energies of l-10 eV.
15 September 1981
$Ar-Kr v=
0
+H2
I
2
3
H;tAr -ArH++H 4
ECM
0
I
2
3
4 Ect.4
70 50-
9,. Experimental The instrumentation used in this work has been described in detail elsewhere [15]. Briefly, it consists of a He discharge lamp (Hopfield continuum), vacuum ultraviolet monochromator, molecular beam photoionization source for g, radiofrequency octopole ion guides operated at 15 MHz, a scattering cell, quadrupole mass spectrometer and Daly detector_ H$ ions are formed within the ion guides by photoionization of a supersonic beam of Hz. Photoionization with wavelengthselected photons permits generation of an ion beam having a well-defined distribution of vibrational states. The ions are then guided through a differentially pumped region, accelerated to the desired collision energy, and passed through a scattering cell. All reactant and product ions are subsequently collected, mass analyzed, and detected. Variation of ionizing wavelength and collision energy allows determination of integral reaction cross sections as a function of vIbrational state and kinetic energy.
3. Results The data reactions (3) and (4) are presented in fig. 1, &I which relative cross sections for u = 04 at several center-of-mass energies are compared_ The details of the data analysis are described elsewhere [ 151_ Briefly, raw experimental data were checked for internal consistency between variable wavelength and variable kinetic energy scans. Vibrational distributions generated at each wavelength were combined with raw cross sections measured to obtain reaction cross sections as a function of vibrational state. The estimated relative error for the proton-transfer cross sections is +5%. For the charge-transfer measurements, the backstreaming of gas from the scattering cell to the photoionization region gives an additional source of error. If the photoionization efficiency curve were constant for Ar over the wavelength range used, the error introduced would be constant. This is not the case [ 161, however, and
30-
r-
I
20NT-5 b IO7532-
Fig. 1. %&rational state dependence of proton- and chargetram&r uoss sections at several kinetic energies 1 V (-), 3V(-•-_),6V(----_)and9V(---)
+ c
our control experiments indicate that appropriate error bars for charge transfer are -tS%, *7% and al 5% for Hz@ = 0, l), H~(u = 2,3) and Hz(u = 4) respectively. Absolute values of cross sections shown in fig_ 2 indicate that at low kinetic energies (< 1 eV) the
eV
2
Hz+ A’r
+Ar
v=o
16.6 V= 164 16.2 160
I
____ IR-_----
4
_____--
3
2py2 _-----~__-_~~---_
2
2b2 _-_=-_-
I--
15s
v=
2P
_-_____
_
_
2p312 I __-~-_-_-__--~___-_
15.6. m
0
____---
______
_---_
Fig. 2. Schematic of energy levels in the H’: + Ar* + Hz systems.
393
Volume 82, number 3
CHEMICAL PHYSICS LETTERS
charge-transfer and proton-transfer cross sections are similar in magnitude, in agreement with previous woik on both H2 t Ar+ and Ar + Hzf_ The most important features to note are (I) the very small charge-transfer cross section for u = 0 compared to u > 0 at all energies, and (2) the strong resemblance between the vibrational dependence of both reactions at kinetic energies >3 eV.
4. Discussion Theoretical treatments of long-range charge transfer [17,1 S] have shown that both energy resonance and favorable Franck-Condon overlaps enhance the process. A diagram of the energy levels appropriate to the H2f + Ar system is shown in fig_ 2. It can be seen that charge transfer from Hi(u > 0) IS near-resonant, while transfer from H$(u = 0) is 0.3 1 eV endothermic. It is only this last state that requires translational to vibrational energy transfer during the collision; indeed the cross sections *are much smaller for this state than for the higher vibrational levels. It can be seen that dependence of the cross sections on translational energy is quite weak despite the existence of energy level mismatches. Peaking of the cross section for u = 2 coincides with both a closely resonant pair of states and the maximum of the Franck-Condon overlap between H2f and H, [ 193 _ These features of the data indicate that near-resonant charge transfer is a predominantly long-range process, in agreement with the molecular beam results. There are significant differences between the results in fig_ 1 and those obtained in photoion-photoelectron coincidence measurements [12,13]. In both of these studies the increase in cross section from u = 0 to u = 1 or 2 is smaller than observed here. At low kinetic energy, the cross section for u = 2 is maximum in our data and those of Tanaka 1133, but the degree of enhancement of u = 1 over u = 0 is larger in the present work, and the fall-off for u = 3 and 4 much smaller. Although our data only exterld up to 10 eV, the rather weak translational energy dependence allows at least a qualitative comparison to be made to the work of Campbell at >20 eV 1123. Their cross sections show amaximumatu=1ratherthanu=2.‘IBisshiftis rather surprising, and not readily explained. Calculations of cross sections at 20 eV do not predict such a shift [12]. 394
15 September 1981
Theoretical treatments of the proton-transfer channel have shown that only Hz f Ar correlates to ArH+. Since H2f(u = 0) has little probability of undergoing charge transfer (a transition to an unreactive surface), it might be expected that direct proton transfer would be more facile for this state than for the higher vibrational states. In fact at ail energies vibration enhances proton transfer_ Furthermore, the strong resemblance between the vibrational dependences for charge and proton transfer at higher energies suggests that above =3 eV, both channels involve passage through a common region of the potential energy surfaces available to the system. Failure to do so may result in largely nonreactive scattering_ Regardless of the details of the collisions, it is not likely that proton transfer only results from simple motion on the Hzf + Ar surface. Comparison of the proton transfer data at 1 eV with those of Tanaka [ 133 indicate that while the observed trends are in agreement, the vibrational enhancement reported here is more dramatic. Poor beam intensities at 0.3 eV preclude measurement of cross sections for comparison to the data of Chupka [ 111, who reported little vibrational effect at that collision energy. In crossed beam experiments [lo], proton transfer from Hz(u < 2) was compared to Hz (all states), and little difference was found. This result is supported by the data in fig. 1, which indicates that the major variation in vibrational effects involve the states u = O-2. The use of vibrationally state-selected ions over a range of kinetic energies has been crucial in revealing new aspects of an often-studied reactive system. Work in progress includes other systems where near-resonant charge transfer is likely to play an important role in the reaction dynamics such as I$ + N2 _
Acknowledgement This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy under Contract Number W-7405-ENG-48.
References [l] P.J. Kuntz and AS. II 68 (1972) 259.
Roach,
J- Chem.
Sot.
Faraday
Trans.
Vohrme 82, number 3
CHEMICAL
[2] S. Chapman and R.K. Preston, J. Chem. Phys. 60 (1974) 650. [3] M. Baer and J.A. Beswick, Phys. Rev. A19 (1979) 1559. [4] B.H. Mahan, Accounts Chem. Res. 8 (197.5) 55. [5] M. Chiang, E.A. GisIason, B H. Mahan, C-W. Tsao and AS. Werner, J. Chem. Phys. 52 (1970) 2698. [6] P M. Hierl, V Pa& and Z. Herman, 3. Chem. Phys. 67 (1977) 2678. [7] K. Tanaka, J. Durup, T Kato and I. Koyano, JXhem. Phys. 73 (1980) 586. [S] R.M BiIotta, F.N. Preuninger and J M. Farrar, Chem. Phys Letters 74 (1980) 95. [9] R.M. BiIotta, F.N. Preuninger and J_hI. Farrar, J. Chem. Phys. 73 (1980) 1637. [lo] R M. Bdotta and J-M. Farrar, J. Chem. Phys. 74 (1981) 1699. [ll] W A. Chupka and M E. Russell, J. Chem. Phys. 49 (1968) 5426
PHYSICS LETI’ERS
15 September 1981
[12] FM. Campbell, R. Browning and C.J. Latimer, J. Phys. B13 (1980) 4257. [ 131 K. Tanaka, T. Karo and I. Koyano, XII JCPZAC, Gatlinburg, Tennessee, July 198 1. [ 141 E. Teloy and D. GerIich, Chem. Phys. 4 (1974) 417. [lS] S.L. Anderson, F-A. HouIe, D. Gerlich and Y-T. Lee, 3. Chem. Phys. (Sept. 1,1981), to be published. [16] R.E. Huffman, Y. Tanaka and J-C. Lan-abee, J. Chem. Phys. 39 (1965) 902. [17] D R Bates and R H.G. Reid, Proc. Roy. Sot. A310 (1969‘) 1.
[18] M-T_ Bowers and T. Su, Advan. Electron. Electron Phys. 34 (1973) 231. [19] J. Berkowrtz and R. Spohr. J. Electron Spectry. 2 (1973) 143.
395