REACTION CROSS SECTIONS OF Ca ( 4 S , 43 p AND 3 STATES ) WITH HALOGENATED COMPOUNDS AND WATER

*Laboratoire de Photophysique MolOculaire du C. N. R. S. UniversitO de Paris-Sud, Bat. 213. 91405 ORSAY + Departamento de Quimica-Fisica, Facultad de Ciencias, Campus Universitario sin Universidad de Castilla-la Mancha, 13071-Ciudad Real. Spain + + Unidad de Ldseres y Haces Moleculares, Instituto Pluridisciplinar, Universidad Complutense, 28040 Madrid, Spain and Departamento de Quimica Fisica, Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid

F. FETHI et al. conditions.They found different reaction products depending upon the electronic state of the Ca atom.A good example of this different dynamical behavior is that of the Ca* + H20 reaction, in which it was found that whereas the metastable Ca(3p) reac- tion produces excited CaOH, the ground state Ca reaction yields predominantly CaO emission.Quantitative data on these excited reactions are not only important, but also necessary to test modern theoretical treatments on reactive collisions so we report here measurements made by above mentioned two techniques, on cross sections and branch- ing ratios for the different exit channels leading to the formation of the CaX product (respectively CaC1, CaBr, and CaI) in ground and first electronically excited states.
Reaction cross-section data of the Ca(1D2)+ H20 system is also reported.

EXPERIMENTAL
The experimental apparatus is a Broida oven already described in details. 16 The Calcium vapor formed in a furnace is entrained with helium by a fast pumping toward the reaction zone Z.By setting a D.C. discharge in the furnace, it can be excited and the fast pumping allows atoms in metastable states 3p and !D to be present in Z.The absolute concentration of these species have been measured by a procedure which is shortly described below and their relative concentration could be varied by changing parameters (pression, temperature, discharge current) whose influence is described in ref. 17.The chemiluminescence spectra are taken with a monochromator Bausch & Lomb (0.6 rn focal length).The L.I.F. is obtained with a home-made dye laser, pumped by a B.M.Industrie YAG laser (30 Hz, 200mJ at 532 nm, 10 ns pulse duration), and observed through the Bausch & Lomb mono- chromator with large slits used as a filter.Standard microcomputer equipment is used to record and data analysis.
The results obtained on both chemiluminescence and laser induced fluorescence are comparable to those obtained in single collision conditions.Indeed, a comparison made on the Mg + C12 system between beam-gas and bulk conditions has shown that the effect of the carrier gas (at a few torr) in a Broida oven is to thermalise the rotational population, with no detectable effect on vibrational and electronic energy distribution of the A2H-XE+-transition. 18A lifetime measurement of the 0-0 band of this transition in CaC1 showed no He pressure dependence in the range of pressure used, and gave 30ns, which is in very good agreement to the already published values. 19As the concentration of reactive species are the same order of magnitude in both experiments, and as the radiative lifetime are short (see comments of the Fig. 3 below), this is not surprising.
The absolute concentration of reactant (Ca3p), Ca(1D)and product (CaX, CaOH) were measured, by comparing their fluorescence desexcitation with the emission of a quartz tungsten halogen lamp (Oriel n6332), whose calibration was made from the NIST traceable calibrated quartz tungsten halogen lamp (type .6315FEL 1000).By measuring the efficiency of our light detection system-solid angle of the collected light through the focusing lens-, we were able to determine, in additon, an absolute value of the cross section involved in the reaction forming CaOH.Another method based on the variation of the fluorescence intensity with the organic reactant con- centration was also used to determine the absolute cross section of CaBr formation.

RESULTS AND DISCUSSION
The observed chemiluminescences are due to the transitions A2I-I-X2 + and B2E +- X2E + of the CaX radical, and of the A-X transition of CaOH.For the three halogenated compounds, energy conservation precludes ground state Ca 41S to give chemiluminescence we have determined the cross section and the branching ratio of the formation of the A and B states, from the two metastable states 3p and 1D.For H20, only the 1D state is involved.

CaOH
We have used results obtained on CaC1 chemiluminescence already published 1 and summed up here; namely: (a) the cross sections of the A and B formation in the reactive collision Ca*+ CC14CaCI(A,B) where Ca* represents Ca(3p) or Ca(1D), have the following values and ty(aP) 0.25 A and (D)= 1.77 A2.
(b) the branching ratio of the A/B CaC1 states formation follows the 3p/1D con- centration, indicating that the B state is preferentially formed by the 1D state, in agreement with the energy scale and with adiabatic correlation rules.
Figure 1 shows the chemiluminescence of CaC1 obtained with CC14 and H20 as reactants, where one can see the CaCI(A-X and B-X transitions), and CaOH(A-X transition) bands.L.I.F.experiments show that actually CaC1 is formed even in the presence of water, but mainly in the ground state (see below).So, essentially the following processes are taking place: F. FETHI et al.

CaCI CHEMILUMINESCENCE
1,e" F-I FT--'iI-I F'I Both reaction are exoergic, as the bond energy of CaC1 is 4.2 ev2, and the bond energy of H20 is 5.1 ev. 21To get the value of the reactive cross section for the formation of CaOH (A),_ we write the following equations: where I(CaC1) represents the total intensity recorded for the A-X and B-X emissions v represents the average relative velocity of the two reactants n(CC14) represents the concentration of CC14 n o and n l, are the concentrations of 1D and 3p states of Ca respectively a D and a, are the cross sections of the two states to form CaCI(A, B) respectively K is a constant, including all the geometrical and detector sensitivity factors, that has be measured.We use equivalent definitions for CaOH A-X emission.

CaC1
Figure 2 displays two excitation spectra obtained with the same reactant as in  The comparison of the two spectra, which show similar intensities for CaC1 signal in both cases, indicates that the CaC1 X state formation is not affected by the presence of H20.It must be mentioned that both CaCI(X) and CaOH(X) are also formed by cascading from A and B states formed by reactions 1 and 2. The meas- ured ratio of the intensities (obtained by the measurements of the surfaces, averaged over some spectra) of the two CaC1 induced fluorescence gives r I CaCl(reaction 1 + 3 + 5)/1CaCl(reaction 5)= 1.08 We have included reaction (1) which, by cascading populates CaC1 ground state.
We then write r [(k a t-k), Ca* + ks,(fa-fa*)]/(ks, fa (6) As Ca*, which represents here Ca(3p) only, is measured to be 2.10 -4 of Ca, one can make the approximation Ca-Ca* Ca, which gives , (.02/8),( + ) (7)  If we estimate the cross section of the reaction (3) to be between 1 and 10 ,i. 2, that implies that the cross section of the reaction (5) is between .003 and .03z.
The cross sections of the formation of A and B states are calculated by the following way: The ratio of the intensities of the B-X to A-X has been measured. 1

I(B-X)/I(A-X) .15 tr(1D){B-X)/[tro(A-X)+ ap(A-X)] (8)
We know that cr3p(A-X)= .25A and %(A-X) + aD(B-X)= 1.77 A? (9)   It is easy to deduce from these three equations that D(A-X) 1.50 ,it OD(B-X) .27/2 In Figure 3 are shown results obtained when the fluorescence induced by laser is dispersed by the monochromator: on the upper spectrum the laser wathelength was set at the maximum of the AI-I3/-X2E + at 6185.4 A; on the lower spectrum, the laser wavelength was set on the maximum of the A2H1/-X2E+-at 6212.7 A. One can see in the first one the two components of the A state, the first being pumped by the laser, and the second being excited by energy transfer or by intramolecular relaxation.In the second there is only the A1/2 component, as predicted by energy considerations.
If one takes the radiative lifetime of the A state z 33 ns, 19 the probability of energy transfer cannot be very large as the two following processes must be considered: CaCI(A2I-I3/) + M -CaCI(A2II1/2) + M CaCI(AH3/2) + He CaCI(AI-I1/) + He + AE (10) (11)   For the first process, where M represents any atomic (Ca, Ca*) or molecular (CC14, CaC1) species, the M concentration is of the order of 1016/m3, and the cross section would be unrealistically large.For the second process, as He pressure is of the order of 1 torr, the calculated cross section would be ca.160 A2, which seems too large.In that case the energy balance involves only AE 70 cm -1 for similar rovib- rational levels, and in any case this observation implies that the intensity of the A21-I1/2-X2Z + transition is always increased by transfer from the A21-I3/2 component.
One can conclude that there is certainly an intramolecular conversion from the A2I-I3/2 to the A2I-I1/2 component.Figure 4 displays the variation of the A2I-Ia/E-X and A21-I1/E-X intensities, excited by the laser light, with the discharge current i.The sharp increase of both intensities when reaches 75 mA is indicative of an increasing of the direct formation of the X state, as the chemiluminescence, which reflects the direct formation of the excited states, does not show this variation at all.The increase can be attributed to the opening of the channel: Ca(3p) + C1-R CaCI(X2E+) + R k6 whose cross section has been estimated to be about 300 times higher than the channel (5) (see above), as the concentration of the 3p state increases with the current in these experimental conditions (see Fig. 5).CURRENT INTENSITY (mA) Figure 4 Intensity dependance (arbitrary units) of the CaC1 A FI -X and A FI -X E Av 0 tran- sitions with the current of the discharge which produces the metastable states of Ca.

CaBr DETERMINATION OF CROSS SECTIONS
As for CaC1, we have determined the cross sections and the branching ratio of the formation of the A and B states of CaBr. Figure 6 displays a typical spectrum of CaBr chemiluminescence obtained by reaction of Ca* with CHBr 3. One can see the A3/2-X and the A1/2-XAv =0 and Av 1 transitions, and the B-X Av =0,-1,-2 transitions.
We can then write similar equations as for CaCl: Let now I'(B-X) be the intensity of the total B-X transition of CaBr.
I(B-X)-n(CHBra),Vr, (no* (ro(a-x))* K if we assume that the B state is preferentially formed by the 1O state, and I(a-X)/n(Rar) Vr* [no,(ro(Caar)], K and we have a similar relation for I(B-X) of CaC1.We form the ratio of the slopes measured for the two emissions, which is equal to 2.27, and we get in a similar manner as above which gives I(B-X)CaBr/I(A-X)CaBr .31 (ro(A-X) + (re(A-X) 1.84 From these relations one gets (rp(A-X) .22A (rD(A-X) 1.62 A A r6sum6 of the different values of the cross sections is given in Table 2.One can see that CaC1 and CaBr have similar values, and that it was not possible to deter- mine the value for the channels leading to ground state products.
with the Figure 5: this is essentially due to energy pooling, as is studied in details in Ref. 17; second-the A-X and B-X intensity emission reaches a maximum for 75 mA, which is almost the same for the 1D Ca state.As the energy pooling, which corresponds to the reaction Ca* + Ca*--+ Ca** + Ca (20)   where Ca** is mainly Ca(3P) is much more efficient for CarD than for Ca3p :2'17 it is very likely that the above mentioned maximum corresponds to that 3P state which can be more reactive than the 1D or 3p. Figure 9 shows the variation of the B-X/A-X intensity ratio with the current: the maximum at 75 mA, which corresponds to the maximum of D concentration, confirms the correlation between B state formation and 1D state concentration already observed for CaC1.
The differences in intensities of the three sequences are due to the band pass of the monochromator, which was set on the Av =0 region.One striking effect of the Calcium afterglow composition on the reaction products appears in Figure 11:  means that only ground states of atomic calcium are reacting), the lower one re- corded with discharge "on" (ground and excited states reacting), but the heating was much higher than usual for both (530 W instead of 410 W, which implies a very high vapor pressure of Ca).Two remarks can be made: (i) a continuum appears in both spectra underlying the Av 0 sequence; (ii) a dramatic decrease of the intensity of the second appears.
The results are obtained using different and independent methods giving similar values, which make us confident on their validity, within an experimental errors, of the order of 10%.As it could be predicted, 1D state is more reactive than 3p, by a factor of 4 to 7. Chemiluminescence is stronger for CaBr than for CaC1, which is confirmed by the higher values of the corresponding cross sections.Laser Induced Fluorescence studies gave some information on the channels leading to the ground states, and on the rotational and vibrational temperatures, but some observations remain to be explained.Further work is now in progress using this technique as well as high-resolution chemiluminescence.

FigureFigure
Figure laLow resolution chemiluminescence spectrum of CaC1 and CaOH obtained by Ca* + CCl4, HaO reactions.The 3p-1s transition at 653 nm has been attenuated.A21I-XaE and BE +-X2E+of CaC1, and Aal-I XaE transitions of CaOH are observed, together with some bands of CaO, and triplet transi- tions of Ca at 610-616 nm.

Figure 1 .
Figure 1.In the upper one, where Ca is excited by the discharge (which means that Ca41S, Ca43p, and Ca31D are present).Both CaC1 and CaOH bands are seen.In the lower one, where only Ca ground state is reacting, only CaC1 bands are seen.The reactions involved are now:Ca* + CC14 CaCI(X) + CC13 k 3

Figure3a
Figure3a Signal observed by the monochromator when only the A2I-Ia/2-X2+ Av-0 transition is excited by the laser.

Figure 5
Figure 5 Intensity dependance (arbitrary units) of Ca transitions 3p-1s at 653nm and 1D-1S at 457 nm, with the discharge current.The intensity of the atomic transitions have been set on the same scale for convenience.

Figure 10 FFigure 11
Figure 10 Laser Induced Fluorescence of the A2I-I-X2E transition of CaBr formed by the Ca* + CHBr reaction.

Table 1
Experimental conditions.
Table2 Values of the cross sections of different channels open by the reactions Ca, Ca* + H20, CC14, CHBr 3.