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Isotope effects for the formation of slow and fast excited hydrogen atoms produced by controlled electron impact on H2 and D2
Volume 75, number 2
CHEMICAL
ISOTOPE EFFECTS FOR THE FORMATION PRODUCED BY CONTROLLED ELECTRON Morrhide
HIGO and Teuchiro
Departttrenr of Molecular hence Hahozakt, Fukuoka 812. Japan Recewed
PHYSICS LETTERS
OF SLOW AND FAST EXCITED IMPACT ON HZ AND D2
15 October
HYDROGEN
1980
ATOMS
OGAWA and Tecl~ttology, Kytrshu Uttrverstty.
4 July 1980, tn final form 14 July 1980
Analysis of the BalmerQ lme taken at hgh resolution clanfies the Isotope effect III the dissoctative cxcltatron of Ha-The tsotope effect IS larger for slow H* atoms (U&OH = 0 4-O 6) than for fast H* atoms and hasalmost no electron energy dependence (X5-100 eV)
I_ Introduction Drssociatron and autororuzation are the primary decay processes for a superexcited molecule Competition between them is an unportant source of the rsotope effect [l] The rsotope effect for the formatron of excited hydrogen atoms produced by controlled electron unpact has been measured on some molecules [24]. However, drssocratrve excitation of these molecules IS expected to proceed vra two or more processes [S] . There has been no measurement of the isotope effect for each process separately. Collisrons between hydride molecules and electrons grve the Balmer lures of H* (D*) atoms. In previous papers [6,7], it was shown that the analysis of the Doppler hne shape taken at hrgh resolution gves the translational energy drstr.bution of the H* atoms. The translational energy drstrrbutron of the H* (n=4) atoms from Ha has three maJor components; their maxuna he at about 0 (slow), and 4 and 8 eV (fast) [5]. These H* atoms are produced by drssocration of bound and repulsive Rydberg states, respectively [S] , and thus they are expected to have drfferent isotope effects fcr theu formation. These Rydberg states are superexcited states. A measurement of the isotope effect For each of the three components separately is desirable but difficult. However, the Balmer lmes of the slow H* atoms (translational energy IS ~0-2 eV) and the fast H* atoms (translational energy above ~2 eV with peaks at 4 and 8 eV) can be separated wrth a resolutron oF~0.03 A.
In the present paper, we have measured the BdmerjJ lmes produced by controlled electron Impact on H, and D2 at hrgh resolution, and obtained isotope effects for the formation of slow and fast H* (II = 4) atoms separately.
2. Experimental The apparatus consrsts of a stamless-steel collision chamber, a hrgh resolutton (==0.03 A) Fabry-Perot interferometer and a photon countrng system. The detarls have been pubbhed elsewhere [7,8]. The H, (99.99999%, Japan Oxygen Company) and D2 (99.5%, Takachrho Chemical Industry) gases were used without further purtficatron. They were bombarded with accelerated and colhmated electrons; an electromagnet was used for the colhmatton of electrons. The Babner-0 line was observed at 90” with respect to the electron beam. The relative intensity of the Balmer lures was obtamed by comparing the areas of Its hrgh resolution spectrum. The data were accumulated and analyzed wtth a microcomputer. Measurements were carried out where the intensity of the BalmerQ hne was proportional to both the gas pressure and the electron-beam current. The Ime shape and the relatrve mtensity were independent of the electron-beam current (20-800 fl), the gas pressure [(O-4-IO) X 10m4 Torr], the magnetic field strength (O-30 G) and the electron-beam diameter (3-10 nun) at an electron energy of 100 eV. 271
3. Results and discussian The Baimer-fl hnes of the W* and D* atoms produced m e--H;! (Dz) coik~ns are dxvtded at the inflectxon points as mdkated by the horizontaf iincs PHI f&. 1 1the upper part denotes the slow N* (II*) atom and the towar part the fast H* CD*) atoms. The ~nfkction pourt corresponds to the mmunum (-2 eV) af
the ~r~s~at~~~a~ energy d~tr~b~t~~~Isotope effects and reIatrve xntensltres are shown rn fig.2_ The relatuxc tntensity of the Balrner lmes of the slaw and fast H* (D*) atoms depends on the electron energy [fig. 2 (middle)] _ In the case of D* atoms, a small overlap between the slc~ and fast IF- atoms is present near the rn~~~rn of the tra~~l~t~~~a~energy d~str~b~ti~~ [9] _However, the ~~~e~a~~ty due to Gnus averiap is smaller than the random experimental fluctuations. The total emrssisn cross sectu3n [3,4&f [fig 2 (tap)] and the relatrve intensity of the Balmer hnes obtarned HI thrs measurement ffig.2 (muddiest gee the isotope effect far the formation of both M* atoms separatefy [fig. 2 (bottom)] as FoUaws
Results at an efectron energy of 100 eV are shown in table f . At an electron energy of 24 eV, the Bahner-fl lmes from Hz and D2 have one component, which COFrespands to to slow W* (D*) atoms [5,9]. So at this electron energy, the rsotope effect for the slow H *
Fsg_ 1. High resolutson co 030-O 032 a> spectra of the Balmer-f3 lmes produced by electron unpact (100 *VI an Hz and 82 The borlzontal hne separates slaw (above) and fast w* (?I= 4) atoms f3xlow)
Fzg. 2- Isotoprt effec% for the f~~rnat~~~of slow and fast H* (n = 4) atoms from Ei2 CD,) Top: rsotope effect on the tota emk~on dtess se&x% of the Bahnex-0 Iine from (a) Karolis and M;utmg [4] and @) ~~~~~~ et af f-319Mlddk- rehtzve tntensrties far the format1011 of sk5w to fast atoms; o fur f-Z* arid 0, far D” atoms Error bars, which bxdxzata the random expenunental, ffuctuations, are omrtted when they are smak Bottom- xsotope effect for the fixation of slow (s) and fast (f3 I-I*atoms, The upper bne of bars indmates the rsotope effect bassxf on Kar~lis and Hartmg f4] and the k~wer lme on Mohhnann aEa!.. [3], 6~) &tape effect for slow f-I* atoms dtrectly obtained from the total emissioncross sectron g3,4’f.
Volume
75, number
2
CHEMICAL
PHYSICS
atoms IS directly obtamed from the total emission cross section [3,4] and IS indicated as a bar with obhque lures (c) 111fig.2 (bottom). Isotope effects on the total emission cross section of H* (D*) atoms by Mohlmann et al. [3] and Karohs and Hartmg [4] are
mconsistent, and accordingly somewhat different ISOrope effects for both the formation of slow and fast H* atoms are obtained. They are shown as bars m fig.:! (bottom). The reason for the inconsistency 1s not clear, and the uncertamty m the isotope effect is mamly due to this difference in the total enusslon
cross section. The isotope effect on the total emission cross section varies with the electron energy [fig 2 (top)] [3, 41. However, the electron energy dependence of the isotope effect for both the formatlon of slow and fast H* atoms IS smaller than the experimental uncertainty. Fig. 2 shows that the electron energy dependence of the isotope effect on the total emission cross section IS due to a variation of the relative cross section for the formation of slow and fast H* (D*) atoms. A more detaded measurement with complete separation of the Balmer hnes mto their components may remove the residual electron energy dependence. It may generally be concluded that the electron energy dependence of the isotope effect on the total enusslon cross sections 1s mamly due to the variation of the relative cross sections of two or more processes. However, a small electron energy dependence may exist near the threshold even for a smgle process due to the dtiference m the width of the Franck-Condon regon. The isotope effect for slow H* atoms 1s larger than for fast H* atoms. The slow H* atoms are produced by direct dissociation of Rydberg states, (1~~) (41), and prelssociation of relevant Rydberg states [5 J. In these R dberg states, dlssociatlon and autolonization to s, J Zl (Isa,), compete wth each other. The wider Franck-Condon repon and the smaller reduced mass of Hz are favorable for dlssoclatlon [3,4]; thus a larger isotope effect for the formation of slow H* atoms is expected. The isotope effect for the formation of fast H* atoms 1s neghgbte on the basis of the total enusslon cross section by Karolis and Harting [4] ; however, it IS small but meaningful on the basis of that by MBhl-
LETTERS
1.5October 1980
mann et al [3] (table I). If this isotope effect is real, it shows that even when H* atoms are formed by dissociation via a repulsive potential curve. the competltlon between dissoclatlon and autoionizatIon to Hz, *Xi (lso,), IS present. Thus fmdmg IS consistent with the fact that the formation ofH* atoms through the repulsive Rydberg states of HCL hx an isotope effect [IO]. The Isotope effect for the formation of H* atoms m each process gives important mformation on the mechanism of H* atom formatlon and competition between dissociation and autoiomzatlon of superehcited states.
Acknowledgement The authors thank Professor Nobuhrko Ishtbashi of the Kyushu University for his encouragement. The present study was partially supported by a Grant-in-Aid for Sclentlfic Research from the Mmistry of Education
References [l] R L. Platzman, Vortex 23 (L962) 372. [2] D.A Vroom and F J de Heer. J. Chcm Phvs. SO (1969) 573, 580; I. FUJI& M Hatada, T. Ogawa and K. Huota, Bull Chem Sot. Japan44 (1971) 1751, I. Tokue, 1. Natuyama and K Kuchdsu, Chem. Phys. Letters 35 (1975) 69; K. Hlrota, M. Hatada and T Ogawa, Intern. J. Radiat. Phys. Chem. 8 (1976) 205. 131 G.R. hlohlmann, F J de Heer and J. Los, Chem. Phys. 25 (1977) 103. [41 C Karohs and E. Hartmg, J Phys. B11 (1978) 357. 151 T. Ogawa and M. Hgo,Chem. Phys. 52 (1980) 55. 161 T. Ogawa and hi Hgo. Chem Phys. Letters 65 (1979) 610. 171 M. Hugo and T. Ogawa.Chem. Phys 44 (1979) 279. I81 M. Higo and T. Ogawa, Rept. Res. Grad Sch. Eng. Sci Kyushu UNV. l(1979) 19. [91 hf. Higo and T. Ogawa, unpubbshed. 1101 T. Ogawa, M. Hso, M Toyoda and N. Ishibasfu, Chem
Letters (1978) 493.
273
CHEMICAL
ISOTOPE EFFECTS FOR THE FORMATION PRODUCED BY CONTROLLED ELECTRON Morrhide
HIGO and Teuchiro
Departttrenr of Molecular hence Hahozakt, Fukuoka 812. Japan Recewed
PHYSICS LETTERS
OF SLOW AND FAST EXCITED IMPACT ON HZ AND D2
15 October
HYDROGEN
1980
ATOMS
OGAWA and Tecl~ttology, Kytrshu Uttrverstty.
4 July 1980, tn final form 14 July 1980
Analysis of the BalmerQ lme taken at hgh resolution clanfies the Isotope effect III the dissoctative cxcltatron of Ha-The tsotope effect IS larger for slow H* atoms (U&OH = 0 4-O 6) than for fast H* atoms and hasalmost no electron energy dependence (X5-100 eV)
I_ Introduction Drssociatron and autororuzation are the primary decay processes for a superexcited molecule Competition between them is an unportant source of the rsotope effect [l] The rsotope effect for the formatron of excited hydrogen atoms produced by controlled electron unpact has been measured on some molecules [24]. However, drssocratrve excitation of these molecules IS expected to proceed vra two or more processes [S] . There has been no measurement of the isotope effect for each process separately. Collisrons between hydride molecules and electrons grve the Balmer lures of H* (D*) atoms. In previous papers [6,7], it was shown that the analysis of the Doppler hne shape taken at hrgh resolution gves the translational energy drstr.bution of the H* atoms. The translational energy drstrrbutron of the H* (n=4) atoms from Ha has three maJor components; their maxuna he at about 0 (slow), and 4 and 8 eV (fast) [5]. These H* atoms are produced by drssocration of bound and repulsive Rydberg states, respectively [S] , and thus they are expected to have drfferent isotope effects fcr theu formation. These Rydberg states are superexcited states. A measurement of the isotope effect For each of the three components separately is desirable but difficult. However, the Balmer lmes of the slow H* atoms (translational energy IS ~0-2 eV) and the fast H* atoms (translational energy above ~2 eV with peaks at 4 and 8 eV) can be separated wrth a resolutron oF~0.03 A.
In the present paper, we have measured the BdmerjJ lmes produced by controlled electron Impact on H, and D2 at hrgh resolution, and obtained isotope effects for the formation of slow and fast H* (II = 4) atoms separately.
2. Experimental The apparatus consrsts of a stamless-steel collision chamber, a hrgh resolutton (==0.03 A) Fabry-Perot interferometer and a photon countrng system. The detarls have been pubbhed elsewhere [7,8]. The H, (99.99999%, Japan Oxygen Company) and D2 (99.5%, Takachrho Chemical Industry) gases were used without further purtficatron. They were bombarded with accelerated and colhmated electrons; an electromagnet was used for the colhmatton of electrons. The Babner-0 line was observed at 90” with respect to the electron beam. The relative intensity of the Balmer lures was obtamed by comparing the areas of Its hrgh resolution spectrum. The data were accumulated and analyzed wtth a microcomputer. Measurements were carried out where the intensity of the BalmerQ hne was proportional to both the gas pressure and the electron-beam current. The Ime shape and the relatrve mtensity were independent of the electron-beam current (20-800 fl), the gas pressure [(O-4-IO) X 10m4 Torr], the magnetic field strength (O-30 G) and the electron-beam diameter (3-10 nun) at an electron energy of 100 eV. 271
3. Results and discussian The Baimer-fl hnes of the W* and D* atoms produced m e--H;! (Dz) coik~ns are dxvtded at the inflectxon points as mdkated by the horizontaf iincs PHI f&. 1 1the upper part denotes the slow N* (II*) atom and the towar part the fast H* CD*) atoms. The ~nfkction pourt corresponds to the mmunum (-2 eV) af
the ~r~s~at~~~a~ energy d~tr~b~t~~~Isotope effects and reIatrve xntensltres are shown rn fig.2_ The relatuxc tntensity of the Balrner lmes of the slaw and fast H* (D*) atoms depends on the electron energy [fig. 2 (middle)] _ In the case of D* atoms, a small overlap between the slc~ and fast IF- atoms is present near the rn~~~rn of the tra~~l~t~~~a~energy d~str~b~ti~~ [9] _However, the ~~~e~a~~ty due to Gnus averiap is smaller than the random experimental fluctuations. The total emrssisn cross sectu3n [3,4&f [fig 2 (tap)] and the relatrve intensity of the Balmer hnes obtarned HI thrs measurement ffig.2 (muddiest gee the isotope effect far the formation of both M* atoms separatefy [fig. 2 (bottom)] as FoUaws
Results at an efectron energy of 100 eV are shown in table f . At an electron energy of 24 eV, the Bahner-fl lmes from Hz and D2 have one component, which COFrespands to to slow W* (D*) atoms [5,9]. So at this electron energy, the rsotope effect for the slow H *
Fsg_ 1. High resolutson co 030-O 032 a> spectra of the Balmer-f3 lmes produced by electron unpact (100 *VI an Hz and 82 The borlzontal hne separates slaw (above) and fast w* (?I= 4) atoms f3xlow)
Fzg. 2- Isotoprt effec% for the f~~rnat~~~of slow and fast H* (n = 4) atoms from Ei2 CD,) Top: rsotope effect on the tota emk~on dtess se&x% of the Bahnex-0 Iine from (a) Karolis and M;utmg [4] and @) ~~~~~~ et af f-319Mlddk- rehtzve tntensrties far the format1011 of sk5w to fast atoms; o fur f-Z* arid 0, far D” atoms Error bars, which bxdxzata the random expenunental, ffuctuations, are omrtted when they are smak Bottom- xsotope effect for the fixation of slow (s) and fast (f3 I-I*atoms, The upper bne of bars indmates the rsotope effect bassxf on Kar~lis and Hartmg f4] and the k~wer lme on Mohhnann aEa!.. [3], 6~) &tape effect for slow f-I* atoms dtrectly obtained from the total emissioncross sectron g3,4’f.
Volume
75, number
2
CHEMICAL
PHYSICS
atoms IS directly obtamed from the total emission cross section [3,4] and IS indicated as a bar with obhque lures (c) 111fig.2 (bottom). Isotope effects on the total emission cross section of H* (D*) atoms by Mohlmann et al. [3] and Karohs and Hartmg [4] are
mconsistent, and accordingly somewhat different ISOrope effects for both the formation of slow and fast H* atoms are obtained. They are shown as bars m fig.:! (bottom). The reason for the inconsistency 1s not clear, and the uncertamty m the isotope effect is mamly due to this difference in the total enusslon
cross section. The isotope effect on the total emission cross section varies with the electron energy [fig 2 (top)] [3, 41. However, the electron energy dependence of the isotope effect for both the formatlon of slow and fast H* atoms IS smaller than the experimental uncertainty. Fig. 2 shows that the electron energy dependence of the isotope effect on the total emission cross section IS due to a variation of the relative cross section for the formation of slow and fast H* (D*) atoms. A more detaded measurement with complete separation of the Balmer hnes mto their components may remove the residual electron energy dependence. It may generally be concluded that the electron energy dependence of the isotope effect on the total enusslon cross sections 1s mamly due to the variation of the relative cross sections of two or more processes. However, a small electron energy dependence may exist near the threshold even for a smgle process due to the dtiference m the width of the Franck-Condon regon. The isotope effect for slow H* atoms 1s larger than for fast H* atoms. The slow H* atoms are produced by direct dissociation of Rydberg states, (1~~) (41), and prelssociation of relevant Rydberg states [5 J. In these R dberg states, dlssociatlon and autolonization to s, J Zl (Isa,), compete wth each other. The wider Franck-Condon repon and the smaller reduced mass of Hz are favorable for dlssoclatlon [3,4]; thus a larger isotope effect for the formation of slow H* atoms is expected. The isotope effect for the formation of fast H* atoms 1s neghgbte on the basis of the total enusslon cross section by Karolis and Harting [4] ; however, it IS small but meaningful on the basis of that by MBhl-
LETTERS
1.5October 1980
mann et al [3] (table I). If this isotope effect is real, it shows that even when H* atoms are formed by dissociation via a repulsive potential curve. the competltlon between dissoclatlon and autoionizatIon to Hz, *Xi (lso,), IS present. Thus fmdmg IS consistent with the fact that the formation ofH* atoms through the repulsive Rydberg states of HCL hx an isotope effect [IO]. The Isotope effect for the formation of H* atoms m each process gives important mformation on the mechanism of H* atom formatlon and competition between dissociation and autoiomzatlon of superehcited states.
Acknowledgement The authors thank Professor Nobuhrko Ishtbashi of the Kyushu University for his encouragement. The present study was partially supported by a Grant-in-Aid for Sclentlfic Research from the Mmistry of Education
References [l] R L. Platzman, Vortex 23 (L962) 372. [2] D.A Vroom and F J de Heer. J. Chcm Phvs. SO (1969) 573, 580; I. FUJI& M Hatada, T. Ogawa and K. Huota, Bull Chem Sot. Japan44 (1971) 1751, I. Tokue, 1. Natuyama and K Kuchdsu, Chem. Phys. Letters 35 (1975) 69; K. Hlrota, M. Hatada and T Ogawa, Intern. J. Radiat. Phys. Chem. 8 (1976) 205. 131 G.R. hlohlmann, F J de Heer and J. Los, Chem. Phys. 25 (1977) 103. [41 C Karohs and E. Hartmg, J Phys. B11 (1978) 357. 151 T. Ogawa and M. Hgo,Chem. Phys. 52 (1980) 55. 161 T. Ogawa and hi Hgo. Chem Phys. Letters 65 (1979) 610. 171 M. Hugo and T. Ogawa.Chem. Phys 44 (1979) 279. I81 M. Higo and T. Ogawa, Rept. Res. Grad Sch. Eng. Sci Kyushu UNV. l(1979) 19. [91 hf. Higo and T. Ogawa, unpubbshed. 1101 T. Ogawa, M. Hso, M Toyoda and N. Ishibasfu, Chem
Letters (1978) 493.
273