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Penning ionization electron spectroscopy.
Volume 4. number 8
CHEWCAL PHYSICSLETTERS
PENNING
IONIZATION
SPECTROSCOPY
OF
ELECTRON
EXCITED
I January
1970
SPECTROSCOPY.
LONG-LIVED
MOLECULAR
STATES
v. EERMAK Institute
of Physical
Chemistry, CzechosEovak Pmgxe, Czechoslovakia
Academy
of Sciences.
Received 17 November 1969 The electron energy distribution in ionization of mercury atoms by excited long-iived St&es of nitrogen molecules was measured and the energy of excited states determined. These are v = 0 and v = I vivibrational levels of the allXg state. brational levels of the E 3X; state and the hi&x
1. LNTRODUCTION Penning Ionization Electron Spectroscopy (PIEIS) is based on the measurement of the energy of electrons released in ionization of atoms
or molecules AB by means of the electronicalLy excited long-lived particles X* [l-5]: X*tAB=AB+tXte. The electron
energy Ee,
(11
is given by
Ee = E(X*) - [IP(AB) + E&LB+)
+ E&lB+
+X,] (2)
where EfX*) is the excitation energy of X, IP the first or higher ionization potentials of AB, E, j the vibrational and rotational excitation of ABC and Ek the total kinetic energy of ABf and X. If the ionization potentials of AB and energetic levels of AB+ ions are to be determined, the excitation energy of X* must be precisely known. For this purpose noble gas atoms in the known metastable states have been successfully applied. However, in considering reaction (1) and eq. (2) another useful spectroscopic application of PIES becomes apparent: it is the detection of energy levels of the neutral particles X* themselves. For this purpose the molecules AB are replaced by an atomic target with known ionization potential. For example, in the reaction N*2+ A = A+ t N2 + e the energy of excited Nz molecules E(N3
= E,
and the distribution
(3) is given by
+ IP(A) t- Ek(N2 f A+)
(4)
of Ee reveals
the
specifically
electronic and vibration& energy of each longlived state provided that the EP of target atoms is sufficiently small to make the ionization possible and that Ek is insignificant. Each energy of released electrons pertains to a specific state of the excited molecule and one can, by varying the energy of the exciting means (e.g. electrons) determine individual e,xcitation functions without the otherwise necessary separation of individual excited states. The excitation energy determined is, of course, that given up in the vertical downward transition, in which vibrationally excited molecular states may be formed. In these respect the PIES of excited neutral states is an analogde of optical emission spectroscopy whereas the determination of ionization potentials and ionic states energy is an analogue of optical absorption spectroscopy. The technique outlined was applied to thz
Ln earlier
publica-
2. RESULTS AND DISCUSSION The integrated stopping potential curve for the electrons released in the system N*-Kg is gived in fig. 1. Taking into account the dr& erence between the true and measured electron energy determined by calibration with metastabIe Ke* atoms (0.18 eV) the curve in fig. 1 indicates the existence of two groups of relezed electrons 515
Volume 4, number 8
CHElMICALPHYSICSLETTERS
1 January 1970
- di dv
I
1
I
2v
Fig. 2. Dia ram of the electronic transitions to and
Fig, I. Electron energy distribution in ionization of Hg atoms by excited long-lived states of Nz. Energy of exciting electrons: 45 eV.
from the E 5 C’ state. Dashed lines: less probable transitions. &l erg-y difference of the ti = 0 and SI= 1 levels: XIX; state 0.29 eV: E %i state 0.27 eV.
with energies of 1.3’7 and 1.08 eV respectively. They must correspond to the excited states giving up the energy of 11.8 and 11.5 eV. The first state is doubtlessly the E “Eg’ state whose long lifetime was demonstrated earlier [S-9] and has been recently measured (.r = 2?0 f 100 cr set) [lo]. The identification of the state Tith less energy is more difficult because the only excited state with known level lying in the energy region of 11.5 eV, the C3Hu (v = 2) state, is probably too short-lived even if subject to perturbation by the 5H state [ll]. A plausible explanation of the lower energy peak in fig. 1 is the following: Excitation to the E 52’ state by electron impact yields mostly vibrat&al level with w = 0. Levels with ZI = 1 and L) = 2 are populated by direct transition, too, but
whose lifetimes shol;ld be long enough because radiation to the lower lying states is forbidden_ ln fact, there are unexplained features in the electron enery loss spectra of Ng at about 11.8 eV [ls] which might belong to au unknown state. To answer these questions experiments with better electron ener resolution are necessary. Besides the E !?C+ state, the highly vibrationally excited levels o% the long-lived a lfIg state (v Z 10) also have enough energy to ionize mercury atoms [12]. The electrons released in this ionization contribute to the steep rise of the curve in fig. 1 at stopping potentials smaller than 0.8 eV.
the determinant population process is cascading
from the higher lying states (e.g. D3Ci). However, in the downward transition to the X’C; ground state, in collisions with Hg atoms, less energetic transfer becomes possible as explained in fig. 2. In fact, by inspection of the published potential energy curves for the X lBi ground state and the E 3Cg’state [12], it becomes evident that the probability of 21’= 1 +u” = 2 and 0’ = 0 + 0” = 1 transitions, releasing energies of 11.56 and 11.58 eV respectively, is high. On the other hand, the more energetic transition 21’= 1 -+ 2)” = 0 (E = 12.14 eV) is certainly not favowed and, accordingly, no peak appears at electron energiek higher than 1.37 eV (fig. 1). The explanation proposed does not exclude that some other electronically excited long-lived state or states may existin the region of 11.5 eV and contribute to the energy peaks, in fig. 1. These might be some other 22 (and %f) states 516
ACKNOWLEDGEMENT I wish to express my tanks to Drs. M. and J. Durup, Laboratoire de Physico-chimie des Rayonnements, Orsay, for many helpful discussions. REFERENCES [l] V. Eerm&, J. Chem. Phys. 44 (1966) 3’774. 3781. [Z] V. ?!ermak, Collection Czech. Chem. Commun. 33 (19:8) 2?39. [3] V. Cermak, Chem. Phys. Letters 2 (1968) 359. [4] V. Fuchs and A. Niehaus, Phys. Rev. Letters 21 (1968) 1136. [5] H. Hotop and A. Niehaus, Chem. Phys. Letters 3 (1969) 687. [6] J.OImsted, A. S-Newton and K. Street, J. Chem. Phps. 42, (196.5)2321. [7] V. Cermak, d. Chem. Phys. 44 (1966) 1318. [8] R. Clampitt and A.S.Newton, (1969) 1997.
J. Chem. Phys. 50
Volume 4, number 8
CHEMICAL PHYSICS LETTERS
[9J li. S. Freund, 3. Chem. Phys. 51 (1969) 1979. [lOJ R. S. Frennd, J. Chem. Phys. 50 (1969) 3734. 111J P. K. Carroli and R. S. MuIiiken, J. Chem. Phys. 43 (1965) 2170.
1 January
1121 R. F. R. GiLmore, J. Qmnt. Spectry. Radiative Transfer 5 (1965) 369. [13] A. Skerbele, M. A. Dillon and E. N. Lassettre, Chem. Phys. 46 (x967) 4261.
1970
J.
517
CHEWCAL PHYSICSLETTERS
PENNING
IONIZATION
SPECTROSCOPY
OF
ELECTRON
EXCITED
I January
1970
SPECTROSCOPY.
LONG-LIVED
MOLECULAR
STATES
v. EERMAK Institute
of Physical
Chemistry, CzechosEovak Pmgxe, Czechoslovakia
Academy
of Sciences.
Received 17 November 1969 The electron energy distribution in ionization of mercury atoms by excited long-iived St&es of nitrogen molecules was measured and the energy of excited states determined. These are v = 0 and v = I vivibrational levels of the allXg state. brational levels of the E 3X; state and the hi&x
1. LNTRODUCTION Penning Ionization Electron Spectroscopy (PIEIS) is based on the measurement of the energy of electrons released in ionization of atoms
or molecules AB by means of the electronicalLy excited long-lived particles X* [l-5]: X*tAB=AB+tXte. The electron
energy Ee,
(11
is given by
Ee = E(X*) - [IP(AB) + E&LB+)
+ E&lB+
+X,] (2)
where EfX*) is the excitation energy of X, IP the first or higher ionization potentials of AB, E, j the vibrational and rotational excitation of ABC and Ek the total kinetic energy of ABf and X. If the ionization potentials of AB and energetic levels of AB+ ions are to be determined, the excitation energy of X* must be precisely known. For this purpose noble gas atoms in the known metastable states have been successfully applied. However, in considering reaction (1) and eq. (2) another useful spectroscopic application of PIES becomes apparent: it is the detection of energy levels of the neutral particles X* themselves. For this purpose the molecules AB are replaced by an atomic target with known ionization potential. For example, in the reaction N*2+ A = A+ t N2 + e the energy of excited Nz molecules E(N3
= E,
and the distribution
(3) is given by
+ IP(A) t- Ek(N2 f A+)
(4)
of Ee reveals
the
specifically
electronic and vibration& energy of each longlived state provided that the EP of target atoms is sufficiently small to make the ionization possible and that Ek is insignificant. Each energy of released electrons pertains to a specific state of the excited molecule and one can, by varying the energy of the exciting means (e.g. electrons) determine individual e,xcitation functions without the otherwise necessary separation of individual excited states. The excitation energy determined is, of course, that given up in the vertical downward transition, in which vibrationally excited molecular states may be formed. In these respect the PIES of excited neutral states is an analogde of optical emission spectroscopy whereas the determination of ionization potentials and ionic states energy is an analogue of optical absorption spectroscopy. The technique outlined was applied to thz
Ln earlier
publica-
2. RESULTS AND DISCUSSION The integrated stopping potential curve for the electrons released in the system N*-Kg is gived in fig. 1. Taking into account the dr& erence between the true and measured electron energy determined by calibration with metastabIe Ke* atoms (0.18 eV) the curve in fig. 1 indicates the existence of two groups of relezed electrons 515
Volume 4, number 8
CHElMICALPHYSICSLETTERS
1 January 1970
- di dv
I
1
I
2v
Fig. 2. Dia ram of the electronic transitions to and
Fig, I. Electron energy distribution in ionization of Hg atoms by excited long-lived states of Nz. Energy of exciting electrons: 45 eV.
from the E 5 C’ state. Dashed lines: less probable transitions. &l erg-y difference of the ti = 0 and SI= 1 levels: XIX; state 0.29 eV: E %i state 0.27 eV.
with energies of 1.3’7 and 1.08 eV respectively. They must correspond to the excited states giving up the energy of 11.8 and 11.5 eV. The first state is doubtlessly the E “Eg’ state whose long lifetime was demonstrated earlier [S-9] and has been recently measured (.r = 2?0 f 100 cr set) [lo]. The identification of the state Tith less energy is more difficult because the only excited state with known level lying in the energy region of 11.5 eV, the C3Hu (v = 2) state, is probably too short-lived even if subject to perturbation by the 5H state [ll]. A plausible explanation of the lower energy peak in fig. 1 is the following: Excitation to the E 52’ state by electron impact yields mostly vibrat&al level with w = 0. Levels with ZI = 1 and L) = 2 are populated by direct transition, too, but
whose lifetimes shol;ld be long enough because radiation to the lower lying states is forbidden_ ln fact, there are unexplained features in the electron enery loss spectra of Ng at about 11.8 eV [ls] which might belong to au unknown state. To answer these questions experiments with better electron ener resolution are necessary. Besides the E !?C+ state, the highly vibrationally excited levels o% the long-lived a lfIg state (v Z 10) also have enough energy to ionize mercury atoms [12]. The electrons released in this ionization contribute to the steep rise of the curve in fig. 1 at stopping potentials smaller than 0.8 eV.
the determinant population process is cascading
from the higher lying states (e.g. D3Ci). However, in the downward transition to the X’C; ground state, in collisions with Hg atoms, less energetic transfer becomes possible as explained in fig. 2. In fact, by inspection of the published potential energy curves for the X lBi ground state and the E 3Cg’state [12], it becomes evident that the probability of 21’= 1 +u” = 2 and 0’ = 0 + 0” = 1 transitions, releasing energies of 11.56 and 11.58 eV respectively, is high. On the other hand, the more energetic transition 21’= 1 -+ 2)” = 0 (E = 12.14 eV) is certainly not favowed and, accordingly, no peak appears at electron energiek higher than 1.37 eV (fig. 1). The explanation proposed does not exclude that some other electronically excited long-lived state or states may existin the region of 11.5 eV and contribute to the energy peaks, in fig. 1. These might be some other 22 (and %f) states 516
ACKNOWLEDGEMENT I wish to express my tanks to Drs. M. and J. Durup, Laboratoire de Physico-chimie des Rayonnements, Orsay, for many helpful discussions. REFERENCES [l] V. Eerm&, J. Chem. Phys. 44 (1966) 3’774. 3781. [Z] V. ?!ermak, Collection Czech. Chem. Commun. 33 (19:8) 2?39. [3] V. Cermak, Chem. Phys. Letters 2 (1968) 359. [4] V. Fuchs and A. Niehaus, Phys. Rev. Letters 21 (1968) 1136. [5] H. Hotop and A. Niehaus, Chem. Phys. Letters 3 (1969) 687. [6] J.OImsted, A. S-Newton and K. Street, J. Chem. Phps. 42, (196.5)2321. [7] V. Cermak, d. Chem. Phys. 44 (1966) 1318. [8] R. Clampitt and A.S.Newton, (1969) 1997.
J. Chem. Phys. 50
Volume 4, number 8
CHEMICAL PHYSICS LETTERS
[9J li. S. Freund, 3. Chem. Phys. 51 (1969) 1979. [lOJ R. S. Frennd, J. Chem. Phys. 50 (1969) 3734. 111J P. K. Carroli and R. S. MuIiiken, J. Chem. Phys. 43 (1965) 2170.
1 January
1121 R. F. R. GiLmore, J. Qmnt. Spectry. Radiative Transfer 5 (1965) 369. [13] A. Skerbele, M. A. Dillon and E. N. Lassettre, Chem. Phys. 46 (x967) 4261.
1970
J.
517