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Primitive product angular distribution for the crossed beam reactions of Ba with NO2 and Cl2
Volume 16. number
3
CHEhlICAL
PHYSICS LETTERS
15 October
1972
PRfkf ITIVE PROI1IUC-I ANGULAR DISTRIBUTIONS FOR THE CROSSED BEAM REACTIONS OF Ba WITH NO, AND CI, $ KG. ANLAUFX:, R.B. BERNSTEM and F-3. VAN ITALLlE Chemistry Department, University of I%consin, Madison, Wisconsin 53706, USA
I..&.I-IABERMAN,
Received
17 July 1972
Anyfar distributions of the BuO and &Cl products from tile thermal crossed beam reactions Ba + X02 - BaO + NO and Ba + Cl? H B&t + Ci have been measured. 9aO md BaCl are predominantly “forward-scattered” in the c.m. system, Only a small fraction of the available reaction exoegicity goes into tr3nslational recoil.
Since the pioneering chemifuminescence beam experiments by Zare and coworkers (I--3i, the reactions of the group ifa metals with various molecules have been the subject of considerable attention. Several studies have now been published on the reactive scattering of Ba in crossed molectilar beams [4-61, and further work is in progress in several laboratories [7-g], its in the present investigation, the hope is to gain insight into the “microscopic” reaction mechanism, including the exoergicity partitioning. Product angular distributions for two prototype reactions (Ba+N02+BnOi+N0 and Ba+Clz~BaCl~+ C1) were observed using an existing crossed-beam ap paratus 110, 1 I] empbyirig universal ionizer-mass ftiter detection, modified in the following manner. The primary Ba beam effused from a stainless steel oven provided with a capillary array, enclosed in a liquid N2 (LN2) cooied copper box located 1 I .O cm from the scattering center (SC). The Ba beam was circularly collimated (fwhm divergence angle 2”), and mechanically chopped at 50 Hz. The secondary beam NO, or.Clz (reagent gmde), effused from another stainless steel capillary array located 1.4 cm from the SC, collimated circularly (observed fwhm angle ea. 20’). The final (heated) collimators and the SC were pa&&y enclosed by an LN2 ccoled copper box. $ Supported by NSF Gnnt GP 26014. $$ Present address: Dept..C.R.A.M., Faculte’ des Sciences, Universiti Laval, Q&cc 10. P.P., Cktxda. 442
Typical operating pressures in the main and detector chamber were, respectively,< 5X IO-? and < 1.5X 1O-9 torr. The in-plane mode of detection was employed, the detector being 9.50 cm from the SC. The entrance capillary to the detector subtended ca. 6X 10-j steradians at the SC. Digital pulse counting techniques were used, recording signal pius background and background-only channels and their difference (the net signal being proportional tc product number density)% Fig. 1, panels (a) and (d), displays the experimental results for the two reactions, shown as the points for BaO and BaCl detection respectively. For reasons not fully understood, the data were of relatively low precision, as indicated by the scatter. Therefore an extensive. computational treatment of the data w2s unwarranted. Thus, simple uncoupled cm. angular and recoil velocity functions were assumed. Existing c.m. + lab transformation programs 112, 131 were modified to accord with present conditions (e.g., product signals proportional to number density, both beams maxwellian, no velocity analysis). The best-fit P(Q) and P(E’) c.m. functions are those shown in
* An SSR lnstmments Co. model 1110 digital synchronous computer was used throughout For ce#ai.u of the later experiments, the origiral mass-fdter power supply was replaced by one from Extranuclear Corp., with a high-Q head Far the most reoent experiments. an on-line Digital Gpip ment Corp. PDP 1 l/20 Lc,b Computer assisted in the data acquisition.
Volume 16, num&r
3
CHEMICAL PHYSICS LETTERS
15 October
1972
cm. functions. Clearly these primitive experiments are of insufficient precision to distinguish among these “best-fit” curves, but a wide range of c.m. functions are excluded prima facie by the present observations. From the strongly “forward” angular distributions of the BaQ and BaCl products, it can be concluded
that these reactions are of the “direct” rather than “complex” type, with reaction occurring at large impact parameters (implying large reaction cross sections) [17j. The relative intensities of the integrated product signals from the two reactions suggested that the Ba + NO, + Da0 c NO reaction cross section was
some 2-3 times larger than that for the Ba + Cl, + -0 8&d&=
BaCl + Cl reaction, in qualitative accord with the ratio based on Zare’s estimated reaction cross sections+. From fig. 1, panels (c) and (f), it appears that only a small fraction of the available energy goes into translationd recoil of the BaO (and B&I) product (in
180
or--.‘---‘1
agreement with the inference drawn from the chemiluminescence results [l-3] and the conclusion of Kinsey [ I7 J that the Ba reactions are analogous to the well-studied alkali reactions). Fig. 1. Panels (a), (d): experimental angular distributions (points) versus calculntions based on four “best-fit” c.m. cross section; o most reliabl? experimental points; c, v , A less reliable data. The heavy solid curve (B) represents the preferred calculated fit, using assumed (uncoupled) c.m. functions P@) = exp[-In Xe/H*)2], i.e., gaussian;and P(E’) 0: (E’)l/2esp [l--F/E*], i.e., Boltzmann. Curves (A), (C) represent results of variations on the If* and E* parameters. The dashed cui~e (D) represents the “best-fit” for an assdmedP(E’) = E’exp[$(I-E’/E*)]. For BaO, the parameters E*(kcal[mole), H*(degees) for curves (A)-(D), pane!s (a)(c), are, respectively: (A) 30,7S”, (B) 25, 90”. (C) 20, 110’ znd (II) 13,105’. For BaCl, curves (A)-(D), panels (d)-(f), they are: (A) 6, 75’. (B) 4, 90”, (C) 3. 110’ and (1)) 4, 90”.
panels (b), (c) and (e), (f) for the two respective reac-
The back-calculated lab angular distributions are shown superimposed on the experimenta points in the top panels, identified A-D corresponding to the
The nominal value of gmX [indicated by the short vertical bzr on the abscissa of panels (c) and (f)] for each reaction is calculated taking into account the know-n Ftr and the
available thermodynamic data on do tant molecules (2, 3, 14-161. .
References
111 Ch. Ottinger and R.N. Zare, Chem. Phys. Letters 5 (1970) 243.
(21 C.D. Jonah and R.N. Zare, Chem. F’hys. Letters 9
tiOIlSS.
f$
$ For the Ba + Cl2 reaction, it is of interest to compare the magnitude oc the cross section for the present observed atom-transfer reaction producing BaCI and that of the “associative radiative recombination” reaction yielding BaC12, as observed by Za.re et 31. [l-3]. In the present experiments, B&l2 Product was sought (near centroid angles) but not detected, i.e., its signal was several orders of magnitude iower than that of the BaCl product.
and &t
of the reac-
(1971) 65.
131 CD. Jonah, R.N. Zare andCh.
Ottinger, J. Chem. Phys. 56 (1972) 263. I41 C. Batalli-Cosmovici and K.W. XIicheI, Chem. Phys: Letters 11 (1971) 245. IS1 R.H. Ncynaber, G.D. hlagnuson, SM. TrujiHo and B.F. Meyers, Phys. Rev. SA (1972) 28.5. IsI J. Fricke, B. Kim and W.L. Fite, Abstracts VII ICPEAC, eds. L-M. Branscomb et aL (North-Holland, Amsterdam, 1971) p. 37. 171 C.A. hiims, S. Lin and R.R. Herm, preprint.
443
Volume
16, number
3
CHEMICAL
D.R. Herschbach, private comrwnication. A. Schultz, H.W. Crust and R.N. Zare, preprint R.W. Bickes Jr. and R-B. Bernstein, Rev. Sci. fnstr. 41 (1970) 759. K.G. Anlaui. R.W_ Bickes Jr. and R-R. Bernstein, 3. Chem. Phys. 54 (1971) 3647. T.T. Wamock and R.B. Bernstein, J. Chem. Phys. 49 (1968) 1878; 51 (1969) 4682. K.T. Gilten, I%. D. T&is, Univ. of Wisconsin, WIS‘ICI-380X (1970).
PHYSICS LETTERS
15 October
1972
[14] 6. Henberg, Electronic spectra of F~Iy~tornic molecules (Van Nostrand, Princeton, 1966). [lj] G. Her&erg, Spectra of diatomic molecules {Van Nostrand, Princeton, 1950). [ 161 I>& StulJ andH. Prophet et al., JANAF Thermachemical Tables, NSRDS-NBS 37. United States Department of Commerce Publication, 2nd Ed. (1970). [ 171 3-L. Kinsey, in: 23iennid review of science, technoIo,qy and medicine, Vol. 9 (MTP, Oxford, 1972) ch. 6.
3
CHEhlICAL
PHYSICS LETTERS
15 October
1972
PRfkf ITIVE PROI1IUC-I ANGULAR DISTRIBUTIONS FOR THE CROSSED BEAM REACTIONS OF Ba WITH NO, AND CI, $ KG. ANLAUFX:, R.B. BERNSTEM and F-3. VAN ITALLlE Chemistry Department, University of I%consin, Madison, Wisconsin 53706, USA
I..&.I-IABERMAN,
Received
17 July 1972
Anyfar distributions of the BuO and &Cl products from tile thermal crossed beam reactions Ba + X02 - BaO + NO and Ba + Cl? H B&t + Ci have been measured. 9aO md BaCl are predominantly “forward-scattered” in the c.m. system, Only a small fraction of the available reaction exoegicity goes into tr3nslational recoil.
Since the pioneering chemifuminescence beam experiments by Zare and coworkers (I--3i, the reactions of the group ifa metals with various molecules have been the subject of considerable attention. Several studies have now been published on the reactive scattering of Ba in crossed molectilar beams [4-61, and further work is in progress in several laboratories [7-g], its in the present investigation, the hope is to gain insight into the “microscopic” reaction mechanism, including the exoergicity partitioning. Product angular distributions for two prototype reactions (Ba+N02+BnOi+N0 and Ba+Clz~BaCl~+ C1) were observed using an existing crossed-beam ap paratus 110, 1 I] empbyirig universal ionizer-mass ftiter detection, modified in the following manner. The primary Ba beam effused from a stainless steel oven provided with a capillary array, enclosed in a liquid N2 (LN2) cooied copper box located 1 I .O cm from the scattering center (SC). The Ba beam was circularly collimated (fwhm divergence angle 2”), and mechanically chopped at 50 Hz. The secondary beam NO, or.Clz (reagent gmde), effused from another stainless steel capillary array located 1.4 cm from the SC, collimated circularly (observed fwhm angle ea. 20’). The final (heated) collimators and the SC were pa&&y enclosed by an LN2 ccoled copper box. $ Supported by NSF Gnnt GP 26014. $$ Present address: Dept..C.R.A.M., Faculte’ des Sciences, Universiti Laval, Q&cc 10. P.P., Cktxda. 442
Typical operating pressures in the main and detector chamber were, respectively,< 5X IO-? and < 1.5X 1O-9 torr. The in-plane mode of detection was employed, the detector being 9.50 cm from the SC. The entrance capillary to the detector subtended ca. 6X 10-j steradians at the SC. Digital pulse counting techniques were used, recording signal pius background and background-only channels and their difference (the net signal being proportional tc product number density)% Fig. 1, panels (a) and (d), displays the experimental results for the two reactions, shown as the points for BaO and BaCl detection respectively. For reasons not fully understood, the data were of relatively low precision, as indicated by the scatter. Therefore an extensive. computational treatment of the data w2s unwarranted. Thus, simple uncoupled cm. angular and recoil velocity functions were assumed. Existing c.m. + lab transformation programs 112, 131 were modified to accord with present conditions (e.g., product signals proportional to number density, both beams maxwellian, no velocity analysis). The best-fit P(Q) and P(E’) c.m. functions are those shown in
* An SSR lnstmments Co. model 1110 digital synchronous computer was used throughout For ce#ai.u of the later experiments, the origiral mass-fdter power supply was replaced by one from Extranuclear Corp., with a high-Q head Far the most reoent experiments. an on-line Digital Gpip ment Corp. PDP 1 l/20 Lc,b Computer assisted in the data acquisition.
Volume 16, num&r
3
CHEMICAL PHYSICS LETTERS
15 October
1972
cm. functions. Clearly these primitive experiments are of insufficient precision to distinguish among these “best-fit” curves, but a wide range of c.m. functions are excluded prima facie by the present observations. From the strongly “forward” angular distributions of the BaQ and BaCl products, it can be concluded
that these reactions are of the “direct” rather than “complex” type, with reaction occurring at large impact parameters (implying large reaction cross sections) [17j. The relative intensities of the integrated product signals from the two reactions suggested that the Ba + NO, + Da0 c NO reaction cross section was
some 2-3 times larger than that for the Ba + Cl, + -0 8&d&=
BaCl + Cl reaction, in qualitative accord with the ratio based on Zare’s estimated reaction cross sections+. From fig. 1, panels (c) and (f), it appears that only a small fraction of the available energy goes into translationd recoil of the BaO (and B&I) product (in
180
or--.‘---‘1
agreement with the inference drawn from the chemiluminescence results [l-3] and the conclusion of Kinsey [ I7 J that the Ba reactions are analogous to the well-studied alkali reactions). Fig. 1. Panels (a), (d): experimental angular distributions (points) versus calculntions based on four “best-fit” c.m. cross section; o most reliabl? experimental points; c, v , A less reliable data. The heavy solid curve (B) represents the preferred calculated fit, using assumed (uncoupled) c.m. functions P@) = exp[-In Xe/H*)2], i.e., gaussian;and P(E’) 0: (E’)l/2esp [l--F/E*], i.e., Boltzmann. Curves (A), (C) represent results of variations on the If* and E* parameters. The dashed cui~e (D) represents the “best-fit” for an assdmedP(E’) = E’exp[$(I-E’/E*)]. For BaO, the parameters E*(kcal[mole), H*(degees) for curves (A)-(D), pane!s (a)(c), are, respectively: (A) 30,7S”, (B) 25, 90”. (C) 20, 110’ znd (II) 13,105’. For BaCl, curves (A)-(D), panels (d)-(f), they are: (A) 6, 75’. (B) 4, 90”, (C) 3. 110’ and (1)) 4, 90”.
panels (b), (c) and (e), (f) for the two respective reac-
The back-calculated lab angular distributions are shown superimposed on the experimenta points in the top panels, identified A-D corresponding to the
The nominal value of gmX [indicated by the short vertical bzr on the abscissa of panels (c) and (f)] for each reaction is calculated taking into account the know-n Ftr and the
available thermodynamic data on do tant molecules (2, 3, 14-161. .
References
111 Ch. Ottinger and R.N. Zare, Chem. Phys. Letters 5 (1970) 243.
(21 C.D. Jonah and R.N. Zare, Chem. F’hys. Letters 9
tiOIlSS.
f$
$ For the Ba + Cl2 reaction, it is of interest to compare the magnitude oc the cross section for the present observed atom-transfer reaction producing BaCI and that of the “associative radiative recombination” reaction yielding BaC12, as observed by Za.re et 31. [l-3]. In the present experiments, B&l2 Product was sought (near centroid angles) but not detected, i.e., its signal was several orders of magnitude iower than that of the BaCl product.
and &t
of the reac-
(1971) 65.
131 CD. Jonah, R.N. Zare andCh.
Ottinger, J. Chem. Phys. 56 (1972) 263. I41 C. Batalli-Cosmovici and K.W. XIicheI, Chem. Phys: Letters 11 (1971) 245. IS1 R.H. Ncynaber, G.D. hlagnuson, SM. TrujiHo and B.F. Meyers, Phys. Rev. SA (1972) 28.5. IsI J. Fricke, B. Kim and W.L. Fite, Abstracts VII ICPEAC, eds. L-M. Branscomb et aL (North-Holland, Amsterdam, 1971) p. 37. 171 C.A. hiims, S. Lin and R.R. Herm, preprint.
443
Volume
16, number
3
CHEMICAL
D.R. Herschbach, private comrwnication. A. Schultz, H.W. Crust and R.N. Zare, preprint R.W. Bickes Jr. and R-B. Bernstein, Rev. Sci. fnstr. 41 (1970) 759. K.G. Anlaui. R.W_ Bickes Jr. and R-R. Bernstein, 3. Chem. Phys. 54 (1971) 3647. T.T. Wamock and R.B. Bernstein, J. Chem. Phys. 49 (1968) 1878; 51 (1969) 4682. K.T. Gilten, I%. D. T&is, Univ. of Wisconsin, WIS‘ICI-380X (1970).
PHYSICS LETTERS
15 October
1972
[14] 6. Henberg, Electronic spectra of F~Iy~tornic molecules (Van Nostrand, Princeton, 1966). [lj] G. Her&erg, Spectra of diatomic molecules {Van Nostrand, Princeton, 1950). [ 161 I>& StulJ andH. Prophet et al., JANAF Thermachemical Tables, NSRDS-NBS 37. United States Department of Commerce Publication, 2nd Ed. (1970). [ 171 3-L. Kinsey, in: 23iennid review of science, technoIo,qy and medicine, Vol. 9 (MTP, Oxford, 1972) ch. 6.