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Radiative lifetime measurements for H2 molecule
J.A. Sánchez, J. Campos
To cite this version:
J.A. Sánchez, J. Campos. Radiative lifetime measurements for H2 molecule. Journal de Physique, 1988, 49 (3), pp.445-449. �10.1051/jphys:01988004903044500�. �jpa-00210714�
Radiative lifetime measurements for H2 molecule
J. A. Sánchez and J. Campos
Cátedra de Fisica At6mica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Avda.
Complutense s/n, 28040 Madrid, Spain
(Reçu le 15 juin 1987, révisé le 10 novembre 1987, accepti le 19 novembre 1987)
Résumé. 2014 Nous avons mesuré, par la méthode de coincidences retardées après excitation par électrons, la durée de vie radiative de 17 niveaux correspondant à 7 états électroniques singulets et 3 triplets de la molécule
H2. Nous rapportons les premières mesures pour quatre niveaux de H2 dont les durées de vie sont comprises
entre 34 et 110 ns.
Abstract. 2014 Radiative lifetime for 17 vibrational levels corresponding to 7 singlet and 3 triplet electronic states have been measured by using a delayed-coincidence method. We report first measurements of four
H2 levels of which lifetimes range from 34 to 110 ns.
Classification
Physics Abstracts
34.80G
1. Introduction.
The hydrogen neutral molecule has been extensively
studied since the late 1920’s. Gale et al. [1],
Richardson [2] and Dieke [3] analysed and identified
its spectrum, setting the basis of H2 spectroscopy.
Since then, an increasing interest exists, as confirmed by recent papers [4-5].
Radiative lifetime measurements for H2 molecule
were first reported by Cahill [6] using a delayed-
coincidence method (DCM). This method [7] has
been employed by many other authors with different excitation processes : electron excitation [8-11], com-
bined electron-laser excitation [12], or with selected VUV light from a synchrotron radiation source [13].
Other techniques have also been used : Miller et al.
[14-15] carried out lifetime measurements with a
MOMRIE (microwave optical magnetic resonance
induced by electrons) technique and Chien et al. [16]
used Hanle effect. For triplet states only Eyler et al.
[12] measured systematically the radiative lifetimes of levels with (ls3d) and (ls3s) electronic configur-
ation.
In the present work we have measured radiative lifetimes of some singlet and triplet levels by using a
DCM with electron excitation. On the other hand, only recently extensive calculations of radiative lifetimes for singlet states [17-19] have been re- ported. As can be seen from the work of Glass- Maujean and coworkers, there are few experimental
results to compare with calculations and some dis-
crepancies exist between experimental works.
In the present work our aim is to report new data for singlet and triplet states to compare with avail- able calculations : we give the first measurements for the radiative lifetimes of four H2 levels by DCM.
The dependence of the measured lifetimes on press-
ure and cascading processes has also been studied.
2. Experimental method.
The apparatus has been previously described in several papers [20-23], so only a brief description
will be given here : a pulsed electron beam excited molecules into the levels of interest. The electron energy varied from 10 to 100 eV. Pulse widths were
in the range 200 ns to 1 f.LS with repetition fre- quencies of 100 kHz and 10 kHz respectively, and
the peak current was about 5 mA. Pulse total cut-off time was less than 4 ns. A time-to-amplitude-con-
verter (TAC) of 1 jjbs range was used and a mul-
tichannel analyser accumulated the TAC signals.
Pressure in the collision chamber ranged from 5 to
100 m torr. The decay curves have been fitted by
one or two exponential terms plus a uniform background by standard methods (least squares).
Optical transitions between 4 000 and 6 500 A,
identified from the work of Dieke [3], were studied.
The light from desexcitation was focussed onto the entrance slit of a 0.25 m Jarrell-Ash monochromator
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01988004903044500
446
Table I. - Radiative lifetimes of H2 singlet states.
(a) Non-adiabatic compling of states (b) Adiabatic approximation (c) Positive component (I 1 n: )
(d) Negative component (lIng)
(e) Line blending of I 1 n: and lIng
(T Experimental error 5 % (g) Experimental error 15 % (h) Dieke notation
(’) Other decay components at excitation energy above 25 eV.
with a double grating mounting. In the range 4 000- 4 800 A single photons were detected by a 56 AVP-
RTC photomultiplier tube. The full width at half
maximum (FWHM) resolution was better than 2 A.
In the 4 800-6 500 A region the photomultiplier was
a 56 TVP-RTC type with S-20 spectral response, cooled by dry ice. FWHM was 5 A.
As is well known, the molecular hydrogen spec- trum presents a very open structure without definite bands, appearing more like a complicated and weak
line spectrum. Although a high spectral resolution
would be needed to avoid overlapping of rotational
lines from different electronic states, it can cause a
lack of luminosity. Our aim has been to carry out measurements at low pressure and near the exci- tation threshold conditions. Therefore our optical
set-up was chosen to provide a high luminosity in
order to study the low pressure dependence and the
eventual cascading component evolution with exci- tation energy, with a suitable signal to noise ratio that otherwise would increase the experimental
error. In the present experiment a previous exper- imental analysis was made to avoid measurements at
complicated line blends. Moreover, measurements for the same upper level at different emission
wavelengths were carried out in order to avoid systematic contributions from weak transitions of other states.
Excitation pulse width of 200 ns was of enough length to assure population of most of the levels. For
levels having radiative lifetimes larger than 100 ns,
we employed a pulse width of 1 uS.
3. Results and discussion.
3.1 SINGLET LEVELS. - Radiative lifetimes of some
levels obtained in this work as well as wavelengths of
studied rotational lines are shown in table I. The transitions between electronic states object of this
paper are in the energy level diagram of figure 1.
The pressure in the collision chamber and the electron excitation energy were systematically varied
within the ranges commented above. Experimental
errors were obtained taking into account statistical and systematic ones and are given in table I.
Comparison between our experimental results and
those from Day et al. [9] and Chien et al. [16] are
shown in table I. Ab initio calculation of radiative lifetimes using adiabatic approximation and non-
adiabatic coupling of double minimum states (EF +
GK + HH) ’Zg+ and I l IIg with J = 1 can also be
seen [17-19]. While for I lng state adiabatic results
are valid, those for I 1IIg were obtained taking into
account angular 1 I: + 1 n: coupling. Positive or
negative component of states is indicated for the observed transitions in table I.
Day et al. [9] lifetime results were obtained from
zero-pressure extrapolation and excitation energy of
Fig. 1. - Energy level diagram for H2 singlet states indicating the employed transitions.
50 eV. Within our pressure range (10-100 m torr) no systematic pressure dependence of these values was
found for any of studied levels near the excitation threshold conditions.
We add new data for three singlet levels :
J ldg (v’ =0),P’.Xgl (v’ = 0 ) and U (v’ = 1). The
first level belongs to the (ls3d) electronic configur-
ation. Our lifetime value is (34.3 ± 2.7) ns, similar to
those of levels corresponding to the same configur-
ation (G 1 Ig’ and 11 Hg). The P 1 Ig’ (v’ = 0 ) level
belongs to the (ls4d) configuration and our lifetime
value is (51.3 ± 4.1) ns. No lifetime calculations exist for these levels. The present result for the lifetime of the doubly-excited state U (v’ = 1) is (110 ± 15) ns.
For Rydberg states good agreement is found with Day et al. previous measurements. Some discrepan-
cies appear with Chien et al. [16] results for I 1 llg
levels.
For doubly excited states the results from Day
et al. are higher than ours.
Theoretical values from Glass-Maujean et al. [17- 19] are in agreement within 20 % in most cases.
The desexcitation curves for all of studied levels
excepting G 1 J;: (v’ = 0) state, showed a single exponential decay suggesting a direct population by
electronic impact. Instead the curves for G 1M+
( v’ = 0) levels measured at 4 628.0 A and 4 634.4 A
448
Fig. 2. - Decay curves for G l.I: (v’ = 0) level at
4 634.5 A, 75 m torr pressure and excitation energy of 75 eV (a), 50 eV (b) and 25 eV (c). In cases (a) and (b)
other component with lifetime of 60 ns is shown. The long-
lived component vanishes in (c) leading to a single exponential of 25.0 ± 1.2 ns. Vertical dotted line shows the end of electron excitation pulse.
for excitation energies above 25 eV showed two
decay components with lifetimes of 25 and 60 ns.
The long-lived component did not appear for exci- tation energies below 25 eV. As reported by Dieke [3] line blending occurs for emission wavelengths corresponding to different upper states for each of
Fig. 3. - Energy level diagram for H2 triplet states indicating the employed transitions.
Table II. - Radiative lifetimes of H2 triplet states.
(a) Other decay components at excitation energy above 14 eV.
the 4 628.0 A and 4 634.4 A lines. Although con-
tributions related to line overlapping can not be
excluded the aforementioned behaviour may be due to cascade population of G 1Mg (v’ = 0) state from
higher ungerade levels. In figure 2 decay curves at
4 634.4 A at 75 m torr pressure, 200 ns pulse width
and 25, 50 and 75 eV excitation energies are shown.
3.2 TRIPLET STATES. - Results for triplet states
from this work and those from references [6, 12, 15]
are shown in table II. The electronic transitions used in the presented work can be seen in the figure 3.
For all of the studied levels no pressure dependence
was found for the measured lifetimes in our exper- imental range.
The desexcitation curves for all studied triplet
levels showed a single exponential decay except for d
3llu (v’ = 0 ) state, which appeared as a sum of two exponential terms with lifetimes of 30 and 80 ns with excitation energies above 14 eV. When the excitation
energy was less than 14 eV only the 30 ns component
appeared in the decay curves. As it was commented
above for G 1Mg: (v’ = 0) this behaviour may be due
to line overlapping or to cascading population from gerade levels.
As can be seen in table II a good agreement is found with levels measured recently [12, 15]. To our
Fig. 4. - Decay curve for u (v’ = 1) level at 5 713.4 Á,
50 m torr pressure and 50 eV excitation energy. The vertical dotted line shows the end of electron excitation
pulse.
knowledge no theoretical results for comparison
exist. Also no experimental results have been re-
ported up to date for u (v’ =1 ) doubly excited state.
Our result for its lifetime is (53.8 ± 4.2) ns (Fig. 4).
Acknowledgments.
One of the authors, J. A. Sánchez, was supported by
a grant from the Comunidad de Madrid. This work
was also supported in part by the spanish DGICYT (project PB 86-0543).
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