Topic number:1
FLOWING AFTERGLOW STUDY OF RECOMBINATION OF HCO+
AND DCO+ IONS WITH ELECTRONS AT THERMAL ENERGIES
T. Kotrík, I. Korolov, R. Plašil, J.Glosík (*)
Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic
[email protected]
Ions HCO+ play important role in dense interstellar molecular clouds. They are formed by
proton transfer in interaction of H3+ ions with CO. Since HCO+ can be easily detected using
radioastronomical techniques, its measured abundance provides a good indication of the
interstellar ionization rate and of the cosmic ray flux. The major destruction mechanism of HCO+
is dissociative recombination. The problem is that there is controversy between theoretical
predictions [1] and experiments and also between some recent experiments (see discussion and
references in [2]). In order to solve the discrepancy we used well defined conditions in Flowing
afterglow experiment (FALP [3]) to remeasure recombination rate coefficients of HCO+ and
DCO+ ions and their temperature dependencies.
In the FALP, high purity He is flowing with high velocity along the Flow tube (L ~ 100 cm,
r = 2.5 cm, pHe ~ 700 – 2500 Pa). Plasma is formed upstream in the microwave discharge and it is
carried along the Flow tube by the buffer gas. Small flow of Ar is added downstream from the
discharge region to remove metastable helium by Penning ionization and to form Ar+ dominated
plasma. When the plasma is already relaxed and cold, CO and H2 (or D2) are introduced and
HCO+ (or DCO+) dominated plasma is formed (for experimental details see ref.. [4]). The decay of
the HCO+ (or DCO+) dominated plasma along the Flow tube is monitored by a Langmuir probe.
The corresponding decay time is 0 – 60 ms. From the measured decay curves the recombination
rate coefficients are calculated using standard technique of “1/ne plot” [4]. Measurements were
carried out at different temperatures and at different densities of reactants to exclude influence of
the ion formation, ion excitation and presence of high energy isomers HOC+ (or DOC+).
6
p = 1200 Pa, T = 180 K,
12
-3
[CO] = 1×10 cm
-7
3 -1
α [10 cm s ]
5
4
3
Fig. 1 Dependence of the recombination
rate coefficient on hydrogen density [H2].
2
1
2
13
3
4
5
-3
H2[10 cm ]
The recombination rate coefficients measured with FALP at different hydrogen densities are
plotted in Fig. 1. When CO and H2 (the discussion for D2 is similar) are introduced to Ar+
dominated plasma both isomers HCO+ and HOC+ are produced. Nevertheless, at used densities of
-7
3 -1
α [10 cm s ]
[H2] and [CO] the high energy isomer HOC+ is converted in ion molecule reactions to HCO+ in a
time scale at least 10 times shorter than typical time of recombination of these ions in the Flow
tube. At used CO and H2 densities we can consider that recombining plasma is dominated by
HCO+ and measured rate coefficient is corresponding to HCO+ ions relaxed to buffer gas
temperature (see discussion in ref. [5]). Measured temperature dependences of the recombination
rate coefficients of the ions HCO+ and DCO+ are plotted in Fig. 2. In Figure is also included the
value of the recombination rate coefficient measured for HCO+ ion using Stationary afterglow
experiment with mass identification of recombining ions (AISA experiment, pressure of He buffer
p = 360 Pa).
1
HCO
DCO
+
our results FALP
Poterya et al.
100
our results FALP
our results AISA
Rowe et al.
Smith et al.
Leu et al.
200
300
+
Laube et al.
Adams et al.
Amano et al.
Poterya et al.
Geppert et al.
400
Fig. 2 Dependence of
the recombination rate
coefficients of ions
HCO+ and DCO+ on
temperature.
For
references see [1,5].
500
T [K]
The measured recombination rate coefficients of HCO+ ion are in very good agreement with
previous experimental results (see discussion in [5] and compilation in refs. [1, 2]). We do not
have agreement with recent calculations [1]. The calculated rate coefficients are in low
temperature region at least by factor of 10 lower than our experimental data. At low temperature
our results for HCO+ and for DCO+ are in contradiction with recent FALP study of Poterya et al.
[2].
This work is a part of the research plan MSM 0021620834 financed by the Ministry of
Education of the Czech Republic and partly was supported by GACR (202/08/H057,
202/07/0495) by GAUK 53607 and GAUK 124707.
Reference
[1] I. A. Mikhailov, V. Kokoouline, A. Larson, S. Tonzani, and C. H. Greene, 2006 Phys. Rev. A 74 032707
[2] V. Poterya, J. L. McLain, N. G. Adams, L. M. Babcovo, 2005 J. Phys. Chem. A 109 7181
[3] J. Glosík, G. Bánó, R. Plašil, A. Luca, P. Zakouril, 1999 Int. J. Mass Spectrom. 189 103
[4] O. Novotny, R. Plasil, A. Pysanenko, I. Korolov, J. Glosik, 2006 J. Phys. B: Atomic Mol. Opt. Phys. 39
2561
[5] R. E. Rosati, M. P. Skrzypkowski, R. Johnsen, M. F. Golde, 2007 J. Chem. Phys. 126 154302