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H+3 vibrational frequencies from ion impact spectroscopy
Volume 5, number 2
CHEMICAL PHYSICS LETTERS
H1; VIBRATIONAL
FREQUENCIES
School of Cltemisfy~,
FROM
ION
1 March 1970
IMPACT
F. PETTY and T. F. MORAN Georgia Institute of Technollogy, Atlanta,
SPECTROSCOPY
Georgia,
USA
Received 29 January 1970 inelastic energy losses in the collisions of Hi with Ne have been measured in a beam apparatus designed to direct rnws analyzed. low energy ion beams! onto target molecules and gc.an the energy, mass,
and angular distributions of charged interaction products. The axperimentti energy loss spectra at smtil
scattering angles correspond to that expected for vibrational excitation of Hi. Frequencies obtained from these inelastic energy loss data are found to agree with the quantum mechanical calculations of Christoffersen and of Schwartz .and Schaad. Transitions involving doubly degenerate Hi bending modes are dominant under the conditions of these experiments.
Hi has been observed in mass spectrometers for many years+[l]$ and the mechanism of its formation by H2 + Hz reactions is well unde+rstood 13-131. Optical measurement of the H3 spectroscopic constants has proved difficult, however collisional inelastic energy loss experiments using ion beams offer a tel:hniaue for determination of these molecular consants. The translational energy loss spectrum of Hi scattered from Ne at a given laboratory scattering angle 0 is given by 1141
Ee’Ei
nt~E 2fi‘lEo case 2 = Al+m * ( M+ In)
(?>z2-&12sin20)+ -
2.0
where E. and E, refer to the energies of the inelastic ah y and elastically scattered ions respectively. E, is the kinetic energy of the incident ion of mass di, m is the mass of the target atom, and AE is the spectroscopic energy loss. For reactants
of known
mass
and velocity
at a given
angle 8, the only unknown quantity in eq. (1) is AE, hence experimental measurement of the in-
elastic energy
loss
(IQ,
-Ei)
in the collisions
Hg+Ne+Hi*+Ne
(2)
allows determination of the internal energy states of the reactant H$ . (E, -Ei) is measured with a $ For references ref. 121. 64
to some of the early
work on Hi see
ENERGY
Fig.
1.5 LOSS:
1.0 Ee
1. H: energy loss spectrum
-
0.5 Ei (cVt
from
0
Hi+Ne
+* +HQ +Ne
interactions at LAB angles of O” and 100. 0 experisum of individual energy 106s curmental points, curves are expected energy loss ves. ----and..... curves corresponding to transitions indicated by respective arrows
tandem mass spectrometric
above.
apparatus [14] which
directs mass analyzed low energy ion beams onto target molecules and scans the energy, mass and angular distribution o+fcharged interaction pr+ducts. The reactant H3 is produced via H2+ )f2 ion-molecu;e reactions in a high pressure ion source. H3 SO formed has been shown to equilibrate with the neutral H2 backgrofnd gas by relatively few collisions [15], hence H3 emerging from the ion source is essentially in the ground
Volume 5. number 2
I htarch 1970
CHEMICALPHYSICSLETTERS Table 1 Computed spectroscopic data Theory
Christoffersen [17] Schwartz and Schaad [18] Ellison et al. [19] Pearson et al. [ZO] Hirschfelder 1211
CI. 12 terms, Sister orbit& CI, Gaussian orbit& Diatomic-inmolecules CI, 18 terms, Gallssian orbitals CI, MO
vibrational state under our experimental conditions. This H; is then accelerated, mass separated by a Nier 60° mass spectrometer, decelerated to 16 eV, and focused into the interaction region containing Ne. The angular, mass and velocity distributions of the interaction products are then measured with an 127O electrostatic sector, mass filter and associated counting equipment that have been described elsewhere [14]. The forward scattered ion current consists primarily of elastically scattered Hi with the inelastically scattered ions appearing as humps on the low ener,7 side of the elastic peak. The inelastic processes are resolved by numerical subtraction of elastically scattered ion beam intensity from total scattered ion beam intensity, and typical results for LAB scattering angles 19= O” and loo are shown by the experimental points in fig. 1. Since the inelastically scattered ions reach the detector with less kinetic energy than those that are elastically scattered, translational-to-internal energy conversion must occur and vibrational and/or rotational excitation of Hi is the only process which can account for the inelastic energy loss. The observed energy loss corresponds to that expected for vibrational transitions. Peak broadening suggests rotational excitation but this cannot be resolved in our present experiments. Similar vibrational and electronic excitations have been observed [16] using diatomic ions at higher kinetic energy. Theoretically calculated [16-201 H$ vibration frequencies are listed in table 1. If these frequencies are used to calculate inelastic energy losses by eq. (l), a direct comparison with experiment can be made. The predicted positions of the energy loss peaks associated with excitation of Hi symmetric stretching A’ and doubly degenerate bending modes E’, given by the arrows in fig. 1, have been calculated using Christoffersen’s frequencies and a harmonic oscillator approximation. The solid lines are
E(au)
R@OhrS)
fJJo(cm_1) -A’(stretch} E’@end) 2790 3354
-1.3326
1.6575
-1.33764
1.6504
3301
___-
-1.357
1.76
3450
2330
-1.3185
1.66
3610
4440
-1.293
1.79
1550
1100
obtained by summing the individual transition curves, which are the dotted and dashed lines. Specific excitationsappear as rather broad
peaks due to thermal movement of Ne target gas and the energy spread of the incident IG beam, since the energy resolution must be experimentally sacrificed for sufficient beam intensity. Experimental energy loss spectra presented as the open circles in fig. I for 0 = O” and 1Oo (0.4O acceptance angle) are to be compared+with the solid lines calculated from theoretical HQ frequencies. Attempts to fit the experimental energy loss data have FUled out all sets of frequencies in table 1 other than Christof-
fersen’s, since the other calculations predict maxima and minima inconsistent with OX data. Higher vibrational transitions predominate when Hz is scattered to larger angles giving indication of more violent encounters at smaller impact parameters, as shown by comparison of the O” and loo data. The area under an individual curve is proportional to the probability of the corresponding transitions. Our best fit to the experimental data at O” is obtained with the ratio of curve areas correspodning to the approximate relative transition probabilities - E’: PO __I = = 0.0’7; A’: PO-l= = 0.50, PO+ 2 = 0.33, Pod3 = 0.06, Po-,2 = 0.04. Acknowledgment is made to the National Aeronautics and Space Administration for support of this research through NsG-657 and for a predoctoral fellowship award to F.P..
REFERENCES [l] J. J.Thomson, Phil. Msg. 24 (1912) 209. 121D. P. Stevenson and D. 0. Schissler, in: Actions chimiques et btilogiirues des radiations. ed. M. Haissinsky (Masson, Paris, 1361). [3]V.L.Tal'roze,Izv.Akad. NzukSSSRSer. Fiz. 24 (1960) 1001. 65
Volume 5, number 2
CHEMICAL PHYSICS LETTER8
[4] B. G. Reuben and L. Friedman, J. Chem. Phys. 37 (1962) 1636. [S] C. F. Giese and W. B. Maier II, J. Chem. Phys. 3.9 (1963) 739. [6] J.H. Futreii and F. P. Abramson, Advan. Chem. Ser. 58 (1966) 107. [‘i] V. AquiIanti and G. G. Volpi, J. Chem. Phys. 44 (1966) 3574. [8] L. D. Doverspike and R. L. Champion, J. Chem. Phys. 4a (1967) 4718. [9] J. Durup and M. Durup, J. Chim. Phys. 64 (1967) 386. [lo] VI. A. Chupka, M. E. Russell and K. Refaey, J. Chem.
Phys. 48 (1968) 1518.
[ll] R. II. Neynaber and S. M. Trujillo, Phys. Rev. 167 (1968) 63. I121 MI.T_ Bowers, D. D. Elleman and J_ King Jr.. J. Chem. Phys. 50 (1969) 4787. [13] E.W. McDaniel, V.&xmak, A. Dalgarno, E. E.
[14] [15] [IS] [17] [i8]
1 March 1970
Ferguson and L. Friedman, Ion-molecule reactions, (Wiley, New York, 1970). P. C. Cosby and T. F. Moran, J. Chem. Phys., to be published. J. J. LeventhaI and L. Friedman, J. Chem. Phys. 49 (1968) 1974; 50 (1969) 2928. J. H. Moore Jr. and J. P. Doering, Phys. Rev. Letters 23 (1969) 564; J. Chem. Phys., to be published. R.E.Christoffersen, J. Chem. Phys. 41 (1964) 960. M. E.Schwartz and L. J. Schaad, J. Chem. Phys. 47 (1967) 5325.
[is] F. 0. Ellison,
N. T. Huff and J. C. Patel,
Chem. Sot. 85 (1963) 3544.
[ZO] h.G.Pearson,
R.D.Poshusta
and J.C.Browne,
J. Chem. Phya. 44 (1966) 1815. [Zl] J.
0. Hirschfelder,
J. Am.
J, Chem. Phys,
6 (1938) 79%
CHEMICAL PHYSICS LETTERS
H1; VIBRATIONAL
FREQUENCIES
School of Cltemisfy~,
FROM
ION
1 March 1970
IMPACT
F. PETTY and T. F. MORAN Georgia Institute of Technollogy, Atlanta,
SPECTROSCOPY
Georgia,
USA
Received 29 January 1970 inelastic energy losses in the collisions of Hi with Ne have been measured in a beam apparatus designed to direct rnws analyzed. low energy ion beams! onto target molecules and gc.an the energy, mass,
and angular distributions of charged interaction products. The axperimentti energy loss spectra at smtil
scattering angles correspond to that expected for vibrational excitation of Hi. Frequencies obtained from these inelastic energy loss data are found to agree with the quantum mechanical calculations of Christoffersen and of Schwartz .and Schaad. Transitions involving doubly degenerate Hi bending modes are dominant under the conditions of these experiments.
Hi has been observed in mass spectrometers for many years+[l]$ and the mechanism of its formation by H2 + Hz reactions is well unde+rstood 13-131. Optical measurement of the H3 spectroscopic constants has proved difficult, however collisional inelastic energy loss experiments using ion beams offer a tel:hniaue for determination of these molecular consants. The translational energy loss spectrum of Hi scattered from Ne at a given laboratory scattering angle 0 is given by 1141
Ee’Ei
nt~E 2fi‘lEo case 2 = Al+m * ( M+ In)
(?>z2-&12sin20)+ -
2.0
where E. and E, refer to the energies of the inelastic ah y and elastically scattered ions respectively. E, is the kinetic energy of the incident ion of mass di, m is the mass of the target atom, and AE is the spectroscopic energy loss. For reactants
of known
mass
and velocity
at a given
angle 8, the only unknown quantity in eq. (1) is AE, hence experimental measurement of the in-
elastic energy
loss
(IQ,
-Ei)
in the collisions
Hg+Ne+Hi*+Ne
(2)
allows determination of the internal energy states of the reactant H$ . (E, -Ei) is measured with a $ For references ref. 121. 64
to some of the early
work on Hi see
ENERGY
Fig.
1.5 LOSS:
1.0 Ee
1. H: energy loss spectrum
-
0.5 Ei (cVt
from
0
Hi+Ne
+* +HQ +Ne
interactions at LAB angles of O” and 100. 0 experisum of individual energy 106s curmental points, curves are expected energy loss ves. ----and..... curves corresponding to transitions indicated by respective arrows
tandem mass spectrometric
above.
apparatus [14] which
directs mass analyzed low energy ion beams onto target molecules and scans the energy, mass and angular distribution o+fcharged interaction pr+ducts. The reactant H3 is produced via H2+ )f2 ion-molecu;e reactions in a high pressure ion source. H3 SO formed has been shown to equilibrate with the neutral H2 backgrofnd gas by relatively few collisions [15], hence H3 emerging from the ion source is essentially in the ground
Volume 5. number 2
I htarch 1970
CHEMICALPHYSICSLETTERS Table 1 Computed spectroscopic data Theory
Christoffersen [17] Schwartz and Schaad [18] Ellison et al. [19] Pearson et al. [ZO] Hirschfelder 1211
CI. 12 terms, Sister orbit& CI, Gaussian orbit& Diatomic-inmolecules CI, 18 terms, Gallssian orbitals CI, MO
vibrational state under our experimental conditions. This H; is then accelerated, mass separated by a Nier 60° mass spectrometer, decelerated to 16 eV, and focused into the interaction region containing Ne. The angular, mass and velocity distributions of the interaction products are then measured with an 127O electrostatic sector, mass filter and associated counting equipment that have been described elsewhere [14]. The forward scattered ion current consists primarily of elastically scattered Hi with the inelastically scattered ions appearing as humps on the low ener,7 side of the elastic peak. The inelastic processes are resolved by numerical subtraction of elastically scattered ion beam intensity from total scattered ion beam intensity, and typical results for LAB scattering angles 19= O” and loo are shown by the experimental points in fig. 1. Since the inelastically scattered ions reach the detector with less kinetic energy than those that are elastically scattered, translational-to-internal energy conversion must occur and vibrational and/or rotational excitation of Hi is the only process which can account for the inelastic energy loss. The observed energy loss corresponds to that expected for vibrational transitions. Peak broadening suggests rotational excitation but this cannot be resolved in our present experiments. Similar vibrational and electronic excitations have been observed [16] using diatomic ions at higher kinetic energy. Theoretically calculated [16-201 H$ vibration frequencies are listed in table 1. If these frequencies are used to calculate inelastic energy losses by eq. (l), a direct comparison with experiment can be made. The predicted positions of the energy loss peaks associated with excitation of Hi symmetric stretching A’ and doubly degenerate bending modes E’, given by the arrows in fig. 1, have been calculated using Christoffersen’s frequencies and a harmonic oscillator approximation. The solid lines are
E(au)
R@OhrS)
fJJo(cm_1) -A’(stretch} E’@end) 2790 3354
-1.3326
1.6575
-1.33764
1.6504
3301
___-
-1.357
1.76
3450
2330
-1.3185
1.66
3610
4440
-1.293
1.79
1550
1100
obtained by summing the individual transition curves, which are the dotted and dashed lines. Specific excitationsappear as rather broad
peaks due to thermal movement of Ne target gas and the energy spread of the incident IG beam, since the energy resolution must be experimentally sacrificed for sufficient beam intensity. Experimental energy loss spectra presented as the open circles in fig. I for 0 = O” and 1Oo (0.4O acceptance angle) are to be compared+with the solid lines calculated from theoretical HQ frequencies. Attempts to fit the experimental energy loss data have FUled out all sets of frequencies in table 1 other than Christof-
fersen’s, since the other calculations predict maxima and minima inconsistent with OX data. Higher vibrational transitions predominate when Hz is scattered to larger angles giving indication of more violent encounters at smaller impact parameters, as shown by comparison of the O” and loo data. The area under an individual curve is proportional to the probability of the corresponding transitions. Our best fit to the experimental data at O” is obtained with the ratio of curve areas correspodning to the approximate relative transition probabilities - E’: PO __I = = 0.0’7; A’: PO-l= = 0.50, PO+ 2 = 0.33, Pod3 = 0.06, Po-,2 = 0.04. Acknowledgment is made to the National Aeronautics and Space Administration for support of this research through NsG-657 and for a predoctoral fellowship award to F.P..
REFERENCES [l] J. J.Thomson, Phil. Msg. 24 (1912) 209. 121D. P. Stevenson and D. 0. Schissler, in: Actions chimiques et btilogiirues des radiations. ed. M. Haissinsky (Masson, Paris, 1361). [3]V.L.Tal'roze,Izv.Akad. NzukSSSRSer. Fiz. 24 (1960) 1001. 65
Volume 5, number 2
CHEMICAL PHYSICS LETTER8
[4] B. G. Reuben and L. Friedman, J. Chem. Phys. 37 (1962) 1636. [S] C. F. Giese and W. B. Maier II, J. Chem. Phys. 3.9 (1963) 739. [6] J.H. Futreii and F. P. Abramson, Advan. Chem. Ser. 58 (1966) 107. [‘i] V. AquiIanti and G. G. Volpi, J. Chem. Phys. 44 (1966) 3574. [8] L. D. Doverspike and R. L. Champion, J. Chem. Phys. 4a (1967) 4718. [9] J. Durup and M. Durup, J. Chim. Phys. 64 (1967) 386. [lo] VI. A. Chupka, M. E. Russell and K. Refaey, J. Chem.
Phys. 48 (1968) 1518.
[ll] R. II. Neynaber and S. M. Trujillo, Phys. Rev. 167 (1968) 63. I121 MI.T_ Bowers, D. D. Elleman and J_ King Jr.. J. Chem. Phys. 50 (1969) 4787. [13] E.W. McDaniel, V.&xmak, A. Dalgarno, E. E.
[14] [15] [IS] [17] [i8]
1 March 1970
Ferguson and L. Friedman, Ion-molecule reactions, (Wiley, New York, 1970). P. C. Cosby and T. F. Moran, J. Chem. Phys., to be published. J. J. LeventhaI and L. Friedman, J. Chem. Phys. 49 (1968) 1974; 50 (1969) 2928. J. H. Moore Jr. and J. P. Doering, Phys. Rev. Letters 23 (1969) 564; J. Chem. Phys., to be published. R.E.Christoffersen, J. Chem. Phys. 41 (1964) 960. M. E.Schwartz and L. J. Schaad, J. Chem. Phys. 47 (1967) 5325.
[is] F. 0. Ellison,
N. T. Huff and J. C. Patel,
Chem. Sot. 85 (1963) 3544.
[ZO] h.G.Pearson,
R.D.Poshusta
and J.C.Browne,
J. Chem. Phya. 44 (1966) 1815. [Zl] J.
0. Hirschfelder,
J. Am.
J, Chem. Phys,
6 (1938) 79%