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Laser induced photodissociation of H+2 ions
CHEMICAL
Volume 24, number 4
LASER INDUCED NP.F.B. F.O_M.-lnstituut
PHYSICS
15 February
LETI-ERS
PHOTODISSOCIATION
1974
OF H; IONS
VAN ASSELT, J.G. MAAS and J. LOS
moor Atoom-
en Molecsulfysica.
Received 23
Amsterdam
Wgm., The Netherlands
November 1973
The momentum spectrum of H* ions obtained by photodissociation of a H’: beam with a laser has been measured. The relative distribution over several vibrational levels in the primary beam is calculated.
1. Introduction Photodissociation of H’2 ions has been measured recently by two groups. Von Busch and Dunn [ 11, by using a Hg-Xe lamp, measured total cross sections for photodissociation as a function of the wavelength, ranging from 2472 to 13613 A. On the other hand, Ozenne et al. [2] worked with a ruby laser and measured the energy distribution of the resulting protons in the forward-backward direction of the ion beam. In the present work only one wavelength has been used: 6943 A.’
Experiments with higher resolution on ti2 and also on HD+ and D$ will be published in the future [3]. For a more complete review of the work on r2, which includes our preliminary results, we refer to a . lecture given by Professor J. Durup [4] . In the present experiment we crossed a beam of 10 keV pz ions with a laser beam inside the cavity of an argon ion laser. The momentum spectrum of the protons resulting from the reaction: H$H+H+
Volume 24, number 4
CHEhlICAL PHYSICS LETTERS
15 February 1974
hSO17A
X=4965A
Fig. 2. The apparatus. A: ion source; B: 30” preselection magnet; C: EinzelI lens; D: deflection plates (two pairs for each direction): E: collimation hole with pumping resistance; F: collision chamber with coIlimation hole; C: deflection plates; H: pumping resistance; I: Brewster angk windows; K: deflection plates; L: entrance hole of the magnet; M: analysing magnet; N: entrance slit of the multiplier; P: Bendix electron multiplier; Q: laser. The collision chamber is not used in the present experiment.
in the forward-backward direction of the ion beam was mcorded at constant wavelength for five different laser lines. As shown in fig. 1, the photons are absorbed by the vario& vibrational levels of the electronic ground state (lsu,)of l$. This causes a transition to the first excited state (2pa,), which is repulsive. The excess energy (e;i) above the dissociation limit which depends on the initial vibrational level is released and distributed equally between the two fragments. The direction of the transition moment in this case (CT-Otransition) is parallel to the molecular axis. As a consequence the intensity pattern is proportional to cos2 8, where 0 is the angle between the molecular axis and the polarisation direction of the laser. If the polarisation axis is parallel to the ion beam there will be a maximum in the forward-backward direction (0 =C)_ On the other hand, if the polarisation axis is perpendicular to the beam, the forward-backward intensity will be zero (0 =900). The cross sections for photodissociation of Ht have been calculate$ and tabulated by Dunn [S, 61, for all 19 vibrational levels as a function of *he wavelength. The intensities of the peaks of the separate vibrational levels found m this work are proportional to these cross sections and to the population of eadh particular level.
556 .. .._ _.
X=2765 A
k
10
5
6.~__15._I_ ullpulsra.“,
. 10
S
0
Fig. 3. Momentum distribution of the proton fragments with respect to the center of mass of the Hz molecular ion. The different wavelengths are indicated.
If one accepts from the first Dunn’s tables to be correct, the relative populations for those vibrational levels found at a fixed wavelength can be calculated. This can be done for all the lines available in the argon ion laser. If these values are consistent, Dunn’s tables are correct. In our source, a monoplasmatron, the H; ions are formed by electron bombardment of hydrogen gas. The resulting ions are distributed over all the 19 possible vibrational levels of the electronic ground state. Frequently it is assumed that the population follows a Franck-Condon pattern [7] _
2. Experimental The apparatus used is essentially a mass siectrometer. A sketch.is given in fig. 2. The ions are. formed in the monoplasmatron A and mass selected by a magnet B. The interaction takes place inside-the laser cavity (I), tihere the ion beam’crosses the laser beam in vacuum. In order to.achieve this, the vacuum system is provided with two windows and is placed between the two mirrors of the laser cavity; In this mode of operation the’photon :. density is much higher than in an extra .. .
..
Volume
24, number 4
CHEMICAL PHYSICS
v=7
h(A)
e
t
e
t
e
t
e
t
5145
1.14
1.16
I.32
1.35
1.47
1.51
1.63
1.67
4880
1.28
1.29
1.45
1.48
1.60
1.64
1.76
1.80
4765
1.32
1.35
1.50
1.54
-
1.70
1.83
1.86
4965
1.22
1.24
1.40
1.43
1.56
1.59
1.71
1.75
5017
1.20
1.22
1.37
1.41
1.52
1.57
1.69
1.73
cavity
experiment.
vacuum
This
( 10mg torr)
combined
keeps
Spectra have been recorded for several of the more intense lines of the argon ion laser in the blue and green part of the visible range_ They are reproduced in fig. 3. One can clearly see the shift in the position of the peaks and a remarkable change in the intensity distribution. This last feature agrees at least qualitatively with what one should expect according to Dunn’s tables [6]. The experimentally observed kinetic energy excess cd agrees within 0.05 eV with the theoretically expected values, though our values are consistently low (see table I). The estimated accuracy in the energy is 0.02 eV. Table 2 shows the relative population of the vibrational states, calculated from the measured intensity distributions and Dunn’s pho-
v=9
ll=S
with
the signal
an ultra high
to noise
ratio
high
enough for dc measurement. The laser used is a 4 W Spectra-Physics argon ion laser with an extended cavity _ Working inside the cavity is only possible if the losses introduced in the cavity are very low; as a consequence the windows of the vacuum system have Brewster’s angle with respect to the laser beam. The polarisation is parallel to the ion beam. The resulting protons are analysed by a magnet M and collected on a Bendix electron multiplier. Typical pressures applied in the present experiment
are: source region 5 X 10e6 torr; collision chamber region 5 X IO-8 torr; interaction 1 X 10mg torr.
1974
3. Results and discussion
Table 1 ed values for the peaks found at different wavelengths (e: experimental; t: theoreticai assumingJ = 0). Wavelengths in A, energies in eV, estimated accuracy 0.02 eV V=6
15 February
LEnERS
and analyser region
todissociation cross sections, normalised to u = 7. The agreement for the vibrational distribution obtained
for the different wavelengths is reasonable; especially for levels 6 and 9. The bigger spread for the u = 8 level may be due to convolution with the more intense peak atv=7. In any case we may conclude from these values that in our beam there is a deviation from the FranckCondon pattern, such as should be expected for ions formed by direct electron impact. Our results also devi-
ate from those given by von Busch and Dunn [I]. Nevertheless, the deviations are not very large. An explanation for this feature could be the collision induced dissociation of &_ in the source region where the pressure is relatively high, as we use a mono-
Table 2 Vibrational population other work
for v = 6 to 9 as cnkulated
from Dunn’s tablesand
the measured
intensity
distributions,
compared
with
M-4) ”
5145
4880
4765
4965
5017
Average
FC”)
Dunnb)
Dunnc)
6
1.81
1.83
1.76
1.76
7
1
1
1
1
1.74
1.78
1.42
1.48
1.40
1
1
1
1
1
8
0.61
0.82
-
0.55
0.78
0.69
0.69
0.68
0.65
9
0.4 I
0.42
0.36
0.39
0.36
0.39
0.47
0.46
0.43
a) Franck-Condon population using the Franck-Condon factors calculated in ref. 181. b, Distribution calculated by von Busch and Dunn [ 11 using their formula (12) for the electronic matrix element between H2 and Hf (their table II C). ‘) Calculated by the same authors
by fitting forniula (6) (their table I1 A).
Volume
24. number
4
CHEMICAL
PHYSICS
LETTERS
15 February
1974
Acknowledgement
plasmatron. Calculations done by Peek [V] predict a dependence of the cross section for collision induced dissociation upon the vibrational quantum number. For higher quantum numbers this cross section is larger than for the lower ones. As a consequence higher vibrational levels will be depopulated more strongly than the lower ones. This agrees with the experimental fact that u = 6 is more and u = 9 is less populated than can be expected from the Franck-Condon factors.
References
4. Conclusions
[ 21 J.B. Ozenne, D. Pham and J. Durup. Chem. Phys Letters
We thank Mr. H.H. Holsboer and Mr. Tj van der Hauw for their invaluable technical assistance. This work is supported by F-0-M. with financial support by Z.W.O.
[ I] F. von Busch and G.H. Dunn, Phys
From the results mentioned above we can conclude that Dunn’s tables [6] are consistent. Secondly, we have here available a method for determining the vibrational population in the primary H‘; beam after extraction from the ion source- This provides also a possibility for the measurement
induced dissociation quantum number.
ofcross
as a function
sections
for collision
of the vibrational
Rev. A5 (1972)
1726.
17 (1972) 422. [3] J.B. Ozenne. D. Pham, bl. Tadjeddine and J. Dump. to be
published. 141 J. Durup, 21. Annual Meeting on Mass Spectrometry
and Allied Topics. San Francisco (Mav 1973). ISI GM. Dunn, Phys. Rev. 172 (;968) 1. . [61 G.H. Dunn, J.I.L.A. Report No. 92 (19681, unpublished. 171 G.H. Dunn and B. van ZyI, Phys Rev. 154 (1967) 40. [81 G.H. Dunn. I. Chem. Phys. 44 (1966) 2592. [91 J.M. Peek, Phys. Rev. 140 (1965) All.
Volume 24, number 4
LASER INDUCED NP.F.B. F.O_M.-lnstituut
PHYSICS
15 February
LETI-ERS
PHOTODISSOCIATION
1974
OF H; IONS
VAN ASSELT, J.G. MAAS and J. LOS
moor Atoom-
en Molecsulfysica.
Received 23
Amsterdam
Wgm., The Netherlands
November 1973
The momentum spectrum of H* ions obtained by photodissociation of a H’: beam with a laser has been measured. The relative distribution over several vibrational levels in the primary beam is calculated.
1. Introduction Photodissociation of H’2 ions has been measured recently by two groups. Von Busch and Dunn [ 11, by using a Hg-Xe lamp, measured total cross sections for photodissociation as a function of the wavelength, ranging from 2472 to 13613 A. On the other hand, Ozenne et al. [2] worked with a ruby laser and measured the energy distribution of the resulting protons in the forward-backward direction of the ion beam. In the present work only one wavelength has been used: 6943 A.’
Experiments with higher resolution on ti2 and also on HD+ and D$ will be published in the future [3]. For a more complete review of the work on r2, which includes our preliminary results, we refer to a . lecture given by Professor J. Durup [4] . In the present experiment we crossed a beam of 10 keV pz ions with a laser beam inside the cavity of an argon ion laser. The momentum spectrum of the protons resulting from the reaction: H$H+H+
Volume 24, number 4
CHEhlICAL PHYSICS LETTERS
15 February 1974
hSO17A
X=4965A
Fig. 2. The apparatus. A: ion source; B: 30” preselection magnet; C: EinzelI lens; D: deflection plates (two pairs for each direction): E: collimation hole with pumping resistance; F: collision chamber with coIlimation hole; C: deflection plates; H: pumping resistance; I: Brewster angk windows; K: deflection plates; L: entrance hole of the magnet; M: analysing magnet; N: entrance slit of the multiplier; P: Bendix electron multiplier; Q: laser. The collision chamber is not used in the present experiment.
in the forward-backward direction of the ion beam was mcorded at constant wavelength for five different laser lines. As shown in fig. 1, the photons are absorbed by the vario& vibrational levels of the electronic ground state (lsu,)of l$. This causes a transition to the first excited state (2pa,), which is repulsive. The excess energy (e;i) above the dissociation limit which depends on the initial vibrational level is released and distributed equally between the two fragments. The direction of the transition moment in this case (CT-Otransition) is parallel to the molecular axis. As a consequence the intensity pattern is proportional to cos2 8, where 0 is the angle between the molecular axis and the polarisation direction of the laser. If the polarisation axis is parallel to the ion beam there will be a maximum in the forward-backward direction (0 =C)_ On the other hand, if the polarisation axis is perpendicular to the beam, the forward-backward intensity will be zero (0 =900). The cross sections for photodissociation of Ht have been calculate$ and tabulated by Dunn [S, 61, for all 19 vibrational levels as a function of *he wavelength. The intensities of the peaks of the separate vibrational levels found m this work are proportional to these cross sections and to the population of eadh particular level.
556 .. .._ _.
X=2765 A
k
10
5
6.~__15._I_ ullpulsra.“,
. 10
S
0
Fig. 3. Momentum distribution of the proton fragments with respect to the center of mass of the Hz molecular ion. The different wavelengths are indicated.
If one accepts from the first Dunn’s tables to be correct, the relative populations for those vibrational levels found at a fixed wavelength can be calculated. This can be done for all the lines available in the argon ion laser. If these values are consistent, Dunn’s tables are correct. In our source, a monoplasmatron, the H; ions are formed by electron bombardment of hydrogen gas. The resulting ions are distributed over all the 19 possible vibrational levels of the electronic ground state. Frequently it is assumed that the population follows a Franck-Condon pattern [7] _
2. Experimental The apparatus used is essentially a mass siectrometer. A sketch.is given in fig. 2. The ions are. formed in the monoplasmatron A and mass selected by a magnet B. The interaction takes place inside-the laser cavity (I), tihere the ion beam’crosses the laser beam in vacuum. In order to.achieve this, the vacuum system is provided with two windows and is placed between the two mirrors of the laser cavity; In this mode of operation the’photon :. density is much higher than in an extra .. .
..
Volume
24, number 4
CHEMICAL PHYSICS
v=7
h(A)
e
t
e
t
e
t
e
t
5145
1.14
1.16
I.32
1.35
1.47
1.51
1.63
1.67
4880
1.28
1.29
1.45
1.48
1.60
1.64
1.76
1.80
4765
1.32
1.35
1.50
1.54
-
1.70
1.83
1.86
4965
1.22
1.24
1.40
1.43
1.56
1.59
1.71
1.75
5017
1.20
1.22
1.37
1.41
1.52
1.57
1.69
1.73
cavity
experiment.
vacuum
This
( 10mg torr)
combined
keeps
Spectra have been recorded for several of the more intense lines of the argon ion laser in the blue and green part of the visible range_ They are reproduced in fig. 3. One can clearly see the shift in the position of the peaks and a remarkable change in the intensity distribution. This last feature agrees at least qualitatively with what one should expect according to Dunn’s tables [6]. The experimentally observed kinetic energy excess cd agrees within 0.05 eV with the theoretically expected values, though our values are consistently low (see table I). The estimated accuracy in the energy is 0.02 eV. Table 2 shows the relative population of the vibrational states, calculated from the measured intensity distributions and Dunn’s pho-
v=9
ll=S
with
the signal
an ultra high
to noise
ratio
high
enough for dc measurement. The laser used is a 4 W Spectra-Physics argon ion laser with an extended cavity _ Working inside the cavity is only possible if the losses introduced in the cavity are very low; as a consequence the windows of the vacuum system have Brewster’s angle with respect to the laser beam. The polarisation is parallel to the ion beam. The resulting protons are analysed by a magnet M and collected on a Bendix electron multiplier. Typical pressures applied in the present experiment
are: source region 5 X 10e6 torr; collision chamber region 5 X IO-8 torr; interaction 1 X 10mg torr.
1974
3. Results and discussion
Table 1 ed values for the peaks found at different wavelengths (e: experimental; t: theoreticai assumingJ = 0). Wavelengths in A, energies in eV, estimated accuracy 0.02 eV V=6
15 February
LEnERS
and analyser region
todissociation cross sections, normalised to u = 7. The agreement for the vibrational distribution obtained
for the different wavelengths is reasonable; especially for levels 6 and 9. The bigger spread for the u = 8 level may be due to convolution with the more intense peak atv=7. In any case we may conclude from these values that in our beam there is a deviation from the FranckCondon pattern, such as should be expected for ions formed by direct electron impact. Our results also devi-
ate from those given by von Busch and Dunn [I]. Nevertheless, the deviations are not very large. An explanation for this feature could be the collision induced dissociation of &_ in the source region where the pressure is relatively high, as we use a mono-
Table 2 Vibrational population other work
for v = 6 to 9 as cnkulated
from Dunn’s tablesand
the measured
intensity
distributions,
compared
with
M-4) ”
5145
4880
4765
4965
5017
Average
FC”)
Dunnb)
Dunnc)
6
1.81
1.83
1.76
1.76
7
1
1
1
1
1.74
1.78
1.42
1.48
1.40
1
1
1
1
1
8
0.61
0.82
-
0.55
0.78
0.69
0.69
0.68
0.65
9
0.4 I
0.42
0.36
0.39
0.36
0.39
0.47
0.46
0.43
a) Franck-Condon population using the Franck-Condon factors calculated in ref. 181. b, Distribution calculated by von Busch and Dunn [ 11 using their formula (12) for the electronic matrix element between H2 and Hf (their table II C). ‘) Calculated by the same authors
by fitting forniula (6) (their table I1 A).
Volume
24. number
4
CHEMICAL
PHYSICS
LETTERS
15 February
1974
Acknowledgement
plasmatron. Calculations done by Peek [V] predict a dependence of the cross section for collision induced dissociation upon the vibrational quantum number. For higher quantum numbers this cross section is larger than for the lower ones. As a consequence higher vibrational levels will be depopulated more strongly than the lower ones. This agrees with the experimental fact that u = 6 is more and u = 9 is less populated than can be expected from the Franck-Condon factors.
References
4. Conclusions
[ 21 J.B. Ozenne, D. Pham and J. Durup. Chem. Phys Letters
We thank Mr. H.H. Holsboer and Mr. Tj van der Hauw for their invaluable technical assistance. This work is supported by F-0-M. with financial support by Z.W.O.
[ I] F. von Busch and G.H. Dunn, Phys
From the results mentioned above we can conclude that Dunn’s tables [6] are consistent. Secondly, we have here available a method for determining the vibrational population in the primary H‘; beam after extraction from the ion source- This provides also a possibility for the measurement
induced dissociation quantum number.
ofcross
as a function
sections
for collision
of the vibrational
Rev. A5 (1972)
1726.
17 (1972) 422. [3] J.B. Ozenne. D. Pham, bl. Tadjeddine and J. Dump. to be
published. 141 J. Durup, 21. Annual Meeting on Mass Spectrometry
and Allied Topics. San Francisco (Mav 1973). ISI GM. Dunn, Phys. Rev. 172 (;968) 1. . [61 G.H. Dunn, J.I.L.A. Report No. 92 (19681, unpublished. 171 G.H. Dunn and B. van ZyI, Phys Rev. 154 (1967) 40. [81 G.H. Dunn. I. Chem. Phys. 44 (1966) 2592. [91 J.M. Peek, Phys. Rev. 140 (1965) All.