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Photodissociation of H+2 by monochromatic light with energy analysis of the ejected H+ ions
Volume
17, number 3
CHEMICAL
PHOTODISSOCIATION WITH
ENERGY
PHYSICS
OF
H;
ANALYSIS
1 December
LETTERS
BY
MONOCHROMATIC
OF
THE
EJECTED
1972
LIGHT H+ IONS
J.-B. OZENNE, D. PHAM and J. DURUP Laboratoire
des Collisions
hniques
*, UniversitP
Received
de Paris&d,
914050rsay.
France
14 August 1972
Kinetic energy spectra of H’ ions produced by photodissociation of fast H: ions by a ruby laser beam polarized either parallel or perpendicular to the ion beam are obtained. For the former polarization direction the spectrum shows a structure which reflects the vibrational levels of the primary ions which undergo dissociation.
1. Introduction
Photodissociation
of a diatornic ion by the mono-
chromatic light of a laser will produce fragments with discrete transIational energies Win the center-of-mass
frame as given by the equation W = hv +E””
,
(11
where hv is the phcton energy and E,I’ the (negative) binding energy of the Y” vibronic level of the initial state with respect to the energy of the upper state at infinite
internuclear
In the present paper we report the first results of a kinetic energy analysis of the H’ ions arising from the photodissociation of Hi ions by the light of a ruby laser. H: was chosen because all theoretical data concerning the relevant transition ( lsug + 2~0,) are available [l-3] _ A thorough experimental study of the photodissociation cross section of Hi and Df2 at various waveleng’Js was recently published by von Busch and Dunn [4].
2. Experimental
distance.
Since the light emitted by the laser is highly golartied, the angular distribution of the photefragments will provide clear-cut information on the direction of the dipole moment of the-relevant transition. If the rotation of the molecular ion during its dissociation is negligible the angular distribution of the fragments will be identical to the distribution of the orientations of the internuclear axes to those ions which absorb the light; if not, which may occur in particular when the primary ions are produced not by direct ionization
A sketch of the experimental set-up is given in fig. 1. A beam of Hi ions is produced by ionization of H, by 60 eV electrons in a Nier-type ion source (Atlas AN 4) and accelerated to 4 keV is crossed at right angle by the light beam of a relaxed pulsed ruby laser deliver-
ing about 10 J of 1.79 eV photons in each puke during about 400 ~_csec;a cylindrical lens focusses the laser beam onto the path of the ion beam. The H+ ions aris-
of the parent molecule but by dissociation of a poly-
ing from the photodissociation are collimated, znalysed according to their momentum by the magnetic
atomic ion or by a chemical reaction, the angular distribution cf the fragments will yield indirect informa-
field of a mass spectrometer scattering angle, as described
and collected at zero in previous work on
tion on the dissociation time compared to the rota-
collision-induced dissociation [5] and predissociation [6]
ticn period.
of fast ions. The detector
l
..
Part of the Laboratoire de Physicc-chimie ‘merits, ass&&J with the C.N.R.S.
.422
des Rayonne
is a secondary electron multiplier foliowed by a 100 MHz amplifier and a counter; the gate of the counter is opened by a signal produced by a photodiode when illuminated by the laser beam
Volume 17. num!xr 3
CHEMICAL PHYSICS LETTERS
1 December 1972
Fig. 1. Sketch of the apparatus (the laser beam is actually perpendicular to the plane of tile figure).
and thereafter delayed by a few psec; the gate is shut after a constant counting time of about half a msec; htween two successive laser flashes the background H* ions resulting from dissociation induced by collision on the residual gas are counted during the same time.
Fig. 2. Kinetic energy spectrum of H+ from photodissociation of Ii; by a ruby Iaser beam polarized paralicl to the ion beam. Upper cu~c: total signal (light beam on): lower cun-e: background (light beam off).
3. Results and discussion Fig. 2 shows a typical kinetic energy spectrum of the H’ ions produced during the laser flash and of the collision-induced background; each point of the upper curve corresponds to two flashes; in this experiment the light beam was polarized parallel to the ion beam. The difference between the total signal and the background is plottedin fi,.0 3 for two orientations of the electric field vector of the light beam, namely parallel and perpendicular to the ion beam. The general trends of the spectra are as expected. According to Franck-Condon principle photodissociation by the ruby laser light produces essentially no H+ ions with little kinetic energy, in striking contrast with. collision-induced
dissociation
(lower
curve of fig.
2, in agreement with previous results [S] ).
The fine structure which appears on the spectrum is quite reproducible at least as regards the left-hand side (fragments emitted backwards) where it is better resolved; it clearly corresponds to the successive vibrational levels of Hl (1x1,) which undergo photodissociation. As it should be, this stmcture is best resolved when &e JX! ions are mainly emitted parallel to the flight direction, which occurs when the light ~CUTI is polari.zed parallel to the ion beam, since the dipole
Fig. 3. Kinetic energy !;pcctrum of H+ from photociissociation of Hf by a ruby laser beam: difference bctwcen total signal and background. 0: luigle between the electric field of the light wave and the direction of the ion beam.
moment of the 1scrg+ 2pa, transition is along the internuclear axis. When the light beam is polarized in the perpendicular direction the photo-fragments are emitted mainly at right angles from the parent ion beam, so that a lace part of them is lost and in addition the spectrum is smoothened as may be seen in fig. 3, lower curve; this smoothening arises from the fact that the magnet analyses only the velocity component of the ions parallel to their flight direction. However, the following shortcomings of this first experiment have to be considered: (i) no monitoring of the output
of each laser flash
423
Volume 17, number 3
CHEMICAL PHYSICS LETTERS
was made; the laser appeared to be relatively stable but, e.g.* the fact that the forward peak in fig. 2, upper curve, is lower than the backward peak arises from the occurrence, during this particular run, of a continuous decrease of.the laser output; (ii) to get a high signal we gave the ion beams a not-too-small angular aperture, viz., 4 X 10e5 and 6 X 10-J steradian for the primary and secondary beams, respectively; a higher definition of the secondary ion beam is expected to yield much better resolved spectra. Because of point (ii) and because we did not treat the spectra to take into account the collection efficiency
functions
of the apparatus,
one cannot
from
data accurately calculate the total translational energy of the fragments in the center-of-mass frame, W, which under high angular resolution would be related to the analysing magnetic field N represented on the abscissa axis in figs. 2 and 3 by the equation
1 December 1972
4. Conclusions The results presznted above show the possibilities afforded by this type of experiment. It musi be pointed out that a good signal-to-background ratio (see fig. 2) was obtained without any use of ultra-high vacuum techniques, because of the short duration of the light pulse. Still shorter pulses in the nanosecond range would be interesting for the study of non-linear optical effects which here are negligible (in our experiment the electric field strength associated with the light wave is about IO5 V/cm), but their use would raise difficulties as regards the time resolution of the detector.
the present
W= T@f/H)>”
,
(2)
where To = 4000 eV; therefore we did ncli indicate any energy scale in figs. 2 and 3 and we shall defer the assignment of the vibrational levels until better resolved spectra will be obtained. Indicatively, the top of the peaks of the upper curve of fig. 3 would correspond according to eq. (2) to 0.7 eV whereas from theory [3] the highest peak should arise from the vibrational levlcl v” = 9, leading to 1%’ = 1.06 eV if ignoring rotational energy.
Acknowiedgement The authors
wish to acknowledge
of Mrs. M. Tadjeddine
in some
the collaboration
of the experimental
work.
References
111S. Cohen, J.R. Hiskes and R.J. Riddcll, Phys. Rev. 119 (1960) 102.5.
121D.R. Bates, J. Chem. Phys. 19 (1951) 1122. 131 G.H. Dunn, Phys. Rw. 172 (1968)
1; J.I.L.A.
Report
No. 92 (1968). 141 F. von Busch and G.H. Dunn, Phys. Rev. A5 (1972) 1726. 151 J. Dump, P. Fournier and D. Pham, Intern. J. Mass Spectram. Ion Phyr 2 <1969) 311. [61 P. Foumier, J.-B. Ozennc and J. Durup, J. Chem. Phys. 53 (1971)) 4095.
17, number 3
CHEMICAL
PHOTODISSOCIATION WITH
ENERGY
PHYSICS
OF
H;
ANALYSIS
1 December
LETTERS
BY
MONOCHROMATIC
OF
THE
EJECTED
1972
LIGHT H+ IONS
J.-B. OZENNE, D. PHAM and J. DURUP Laboratoire
des Collisions
hniques
*, UniversitP
Received
de Paris&d,
914050rsay.
France
14 August 1972
Kinetic energy spectra of H’ ions produced by photodissociation of fast H: ions by a ruby laser beam polarized either parallel or perpendicular to the ion beam are obtained. For the former polarization direction the spectrum shows a structure which reflects the vibrational levels of the primary ions which undergo dissociation.
1. Introduction
Photodissociation
of a diatornic ion by the mono-
chromatic light of a laser will produce fragments with discrete transIational energies Win the center-of-mass
frame as given by the equation W = hv +E””
,
(11
where hv is the phcton energy and E,I’ the (negative) binding energy of the Y” vibronic level of the initial state with respect to the energy of the upper state at infinite
internuclear
In the present paper we report the first results of a kinetic energy analysis of the H’ ions arising from the photodissociation of Hi ions by the light of a ruby laser. H: was chosen because all theoretical data concerning the relevant transition ( lsug + 2~0,) are available [l-3] _ A thorough experimental study of the photodissociation cross section of Hi and Df2 at various waveleng’Js was recently published by von Busch and Dunn [4].
2. Experimental
distance.
Since the light emitted by the laser is highly golartied, the angular distribution of the photefragments will provide clear-cut information on the direction of the dipole moment of the-relevant transition. If the rotation of the molecular ion during its dissociation is negligible the angular distribution of the fragments will be identical to the distribution of the orientations of the internuclear axes to those ions which absorb the light; if not, which may occur in particular when the primary ions are produced not by direct ionization
A sketch of the experimental set-up is given in fig. 1. A beam of Hi ions is produced by ionization of H, by 60 eV electrons in a Nier-type ion source (Atlas AN 4) and accelerated to 4 keV is crossed at right angle by the light beam of a relaxed pulsed ruby laser deliver-
ing about 10 J of 1.79 eV photons in each puke during about 400 ~_csec;a cylindrical lens focusses the laser beam onto the path of the ion beam. The H+ ions aris-
of the parent molecule but by dissociation of a poly-
ing from the photodissociation are collimated, znalysed according to their momentum by the magnetic
atomic ion or by a chemical reaction, the angular distribution cf the fragments will yield indirect informa-
field of a mass spectrometer scattering angle, as described
and collected at zero in previous work on
tion on the dissociation time compared to the rota-
collision-induced dissociation [5] and predissociation [6]
ticn period.
of fast ions. The detector
l
..
Part of the Laboratoire de Physicc-chimie ‘merits, ass&&J with the C.N.R.S.
.422
des Rayonne
is a secondary electron multiplier foliowed by a 100 MHz amplifier and a counter; the gate of the counter is opened by a signal produced by a photodiode when illuminated by the laser beam
Volume 17. num!xr 3
CHEMICAL PHYSICS LETTERS
1 December 1972
Fig. 1. Sketch of the apparatus (the laser beam is actually perpendicular to the plane of tile figure).
and thereafter delayed by a few psec; the gate is shut after a constant counting time of about half a msec; htween two successive laser flashes the background H* ions resulting from dissociation induced by collision on the residual gas are counted during the same time.
Fig. 2. Kinetic energy spectrum of H+ from photodissociation of Ii; by a ruby Iaser beam polarized paralicl to the ion beam. Upper cu~c: total signal (light beam on): lower cun-e: background (light beam off).
3. Results and discussion Fig. 2 shows a typical kinetic energy spectrum of the H’ ions produced during the laser flash and of the collision-induced background; each point of the upper curve corresponds to two flashes; in this experiment the light beam was polarized parallel to the ion beam. The difference between the total signal and the background is plottedin fi,.0 3 for two orientations of the electric field vector of the light beam, namely parallel and perpendicular to the ion beam. The general trends of the spectra are as expected. According to Franck-Condon principle photodissociation by the ruby laser light produces essentially no H+ ions with little kinetic energy, in striking contrast with. collision-induced
dissociation
(lower
curve of fig.
2, in agreement with previous results [S] ).
The fine structure which appears on the spectrum is quite reproducible at least as regards the left-hand side (fragments emitted backwards) where it is better resolved; it clearly corresponds to the successive vibrational levels of Hl (1x1,) which undergo photodissociation. As it should be, this stmcture is best resolved when &e JX! ions are mainly emitted parallel to the flight direction, which occurs when the light ~CUTI is polari.zed parallel to the ion beam, since the dipole
Fig. 3. Kinetic energy !;pcctrum of H+ from photociissociation of Hf by a ruby laser beam: difference bctwcen total signal and background. 0: luigle between the electric field of the light wave and the direction of the ion beam.
moment of the 1scrg+ 2pa, transition is along the internuclear axis. When the light beam is polarized in the perpendicular direction the photo-fragments are emitted mainly at right angles from the parent ion beam, so that a lace part of them is lost and in addition the spectrum is smoothened as may be seen in fig. 3, lower curve; this smoothening arises from the fact that the magnet analyses only the velocity component of the ions parallel to their flight direction. However, the following shortcomings of this first experiment have to be considered: (i) no monitoring of the output
of each laser flash
423
Volume 17, number 3
CHEMICAL PHYSICS LETTERS
was made; the laser appeared to be relatively stable but, e.g.* the fact that the forward peak in fig. 2, upper curve, is lower than the backward peak arises from the occurrence, during this particular run, of a continuous decrease of.the laser output; (ii) to get a high signal we gave the ion beams a not-too-small angular aperture, viz., 4 X 10e5 and 6 X 10-J steradian for the primary and secondary beams, respectively; a higher definition of the secondary ion beam is expected to yield much better resolved spectra. Because of point (ii) and because we did not treat the spectra to take into account the collection efficiency
functions
of the apparatus,
one cannot
from
data accurately calculate the total translational energy of the fragments in the center-of-mass frame, W, which under high angular resolution would be related to the analysing magnetic field N represented on the abscissa axis in figs. 2 and 3 by the equation
1 December 1972
4. Conclusions The results presznted above show the possibilities afforded by this type of experiment. It musi be pointed out that a good signal-to-background ratio (see fig. 2) was obtained without any use of ultra-high vacuum techniques, because of the short duration of the light pulse. Still shorter pulses in the nanosecond range would be interesting for the study of non-linear optical effects which here are negligible (in our experiment the electric field strength associated with the light wave is about IO5 V/cm), but their use would raise difficulties as regards the time resolution of the detector.
the present
W= T@f/H)>”
,
(2)
where To = 4000 eV; therefore we did ncli indicate any energy scale in figs. 2 and 3 and we shall defer the assignment of the vibrational levels until better resolved spectra will be obtained. Indicatively, the top of the peaks of the upper curve of fig. 3 would correspond according to eq. (2) to 0.7 eV whereas from theory [3] the highest peak should arise from the vibrational levlcl v” = 9, leading to 1%’ = 1.06 eV if ignoring rotational energy.
Acknowiedgement The authors
wish to acknowledge
of Mrs. M. Tadjeddine
in some
the collaboration
of the experimental
work.
References
111S. Cohen, J.R. Hiskes and R.J. Riddcll, Phys. Rev. 119 (1960) 102.5.
121D.R. Bates, J. Chem. Phys. 19 (1951) 1122. 131 G.H. Dunn, Phys. Rw. 172 (1968)
1; J.I.L.A.
Report
No. 92 (1968). 141 F. von Busch and G.H. Dunn, Phys. Rev. A5 (1972) 1726. 151 J. Dump, P. Fournier and D. Pham, Intern. J. Mass Spectram. Ion Phyr 2 <1969) 311. [61 P. Foumier, J.-B. Ozennc and J. Durup, J. Chem. Phys. 53 (1971)) 4095.