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“Forbidden” transitions in the gas phase paramagnetic resonance spectra of halogen atoms
CHEMICAL PHYSICS L!Zl-ERS
Volume 11, number 3
“FORBIDDEN” TRANSITIONS IN THE GAS PHASE PAIL&MAGNETICRESONANCE SPEClXA OF HALOGEN ATOMS
Received 27 August 197 1
Electron resonance gas phase transitions with a polarization paraUe1 to the externa! maSrwtic fietd are reported * in halogen atoms Atoms are produced inside the ESR cavity by means of a hot-wire method.
1. Introduction
Electron resonance spectra of chlorine f l] , bromine [Z] and iodine [31 atoms in their 2P,,2 ground state have been reported. The observed transitions were alI of the type A&fJ = t 1, AMI = 0 in the high-field quantum number representation. They were induced by the magnetic component of the microwave tieId perpendi~~l~ to the external magnetic field. Transitions arising from the pa&e1 magnetic component of the microwave field have only been detected so far in fluorine atoms [4] _ En ail cases the atoms were produced by a rnicrowave discharge in a gas stream entering the ESR cavity. We wish to report the detection of various “paraG transitions” in halogen atoms obtained by thermolysis.
2, Experimental Halogen atoms were produced by means of thermal dissociation inside a cylindrical, vacuum-tint cavity operating in the T&O1 1 mode at approximately 9180 MHz. A platinum w&e, fitted along the cavity axis and oriented parallel to the external magnetic field, could.be heated electrically. ~e~olysis of ‘chlbrine, bromine and io~~‘v~pour produced strong absorption signals due to halogen atoms. Linewidths were approximately 1,G. Because the atoms are produced in the r&on of the cavity axis, the ex~e~en~ a~~gement is optimized for the detection of $arailel trtisitjons. : . ..
-
‘.‘
3. Theory The spectra of halogen atoms in their 2T?,i2ground state can be described s&isf’ctoriIy by the following hamiltonian, working within the ground&ate manifold:
The three terms represent the Zeemart term sl,, the nudearelectron magnetic hyperfine coughing 5FD and the electric q~d~~~e ~terac~~R Q, respectively. IfA and/or e2@ are large, offdagonal elements ‘,
-. :
:..
.’ :
A Varizn V4500 spectrometer was used in conjunction with a 9 in magnet. The klystron frequency was measured with a Hewlett-Packard 52452, frequency counter in combination with a S25SA frequency converter. The magnetic field values were measured by means of an AEG nuclear magnetic resonance gaussmeter, in conjunction with the frequency counter men~oned above. The pressure in the vacuum system was appro~mately 0.1 torr. The cavity was connected through a liquid-nitrogen trap to a Precision Scientific Company two-stage vacuum pump with a capacity of f 500 ljmin. Low flow rates or no flaw at al.Iproved sufficient to obtain good spectra.
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.
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3
CHEMICAL PHYSICS LEl-iZRS
,’
become significant and mixing of the IZMI,JMJ) levels ,b+oties important. It appears that the terms I-I and (IsJ’)~ in the, hamiltonian give rise to the mixing of drfferent levels:1.7 connacts IIM ;JM;) with ILVI~l,J_M~ 1) levels, and (1.J) a couples luM,,.M,> -withboth IL%f$l,JMJ~l) and liM,+2, JMJT2)_ By .applying these operators, either separately or successively, three types of mixing which may induce transitions with parallei polarization can occur: (a).between IIMI,JMJ>and IIMI+ 1, JMp i ) levels 41J parallel transitions are possible; (b) between IIMI,JMJ)and ILWI+2, JMJT2) levels (U- l)(W-1) parallel transitions, occurring at ap proximately. “half field” are possible; (c) between IMI,JMJ) and ILUI+3,JMJT3) levels (2f-2)(U-2) parallel transitions, occurring at ap proximately “one third field” are possible.
4. Results A computer programme was written to diagonal&e the hamiltonian (,I.), Energy levels for different r-nag netic field values were calculated and transition fields
Tabie 1 Comparison of observed and calculated magnetic field values, and computed intensities, for “half- and third-field” transidons in rz71 atoms. The experimental error in line positions is * 1 G, the spectrometer frequency 9177 MHz Observed
Calculated
Relative intensity
3069.6 2456.1 2307.5 2045.7 1660.2 1852.4 1691.3 1428.3 1379.5 load 466.1
3070.9 2457.2 2308.5 2046.6 1861.1 1853.3 1693.5 1430.4 1381.1 967.6 687.7
4.8 9.1 5.9 4.6 7.2 3.6 1.8 2.7 2.8 0.2 1.0
:
,: ,,-
1379.5G
-H
Fig. 1. Part of the low-field q&rum of “‘1 atoms (spectiometer frequency 9177 MHz).
were obtained by linear interpolation. Relative intensities for the parallel transitions were also computed. For 12JI (nuclear spin S/2) out of 15 + 8 + 3 = 26 possible parallel transitions, 25 were detected, the
missing one being too weak to be observed. The iodine spectrum was calculated, using values for A and e2qQ obtained from atomic-beam work [5], and agJ value obtained from gas phase ESR [3] . Good agreement between calculated and observed line positions was found. Low-field results for iodine are given in table 1, and part of the low-field spectrum in fig. 1. For low magnetic fields, where the Zeeman term is no longer dominant, the mixing of energy levels becomes very extensive and therefore the quantum numbers MI and A/i, can no longer be used. Parallel transitions in chlorine, bromine and iodine at norr& magnetic field values and a complete description of the ESR cavity will be the subject of a separate publication [6].
References [l] V.Beluan-Lopez and H.G.Rob’inson, Phys. Rev. 123 (1961) 161. [2] J.S.M.Harvey, R.A.Kamper andK.R.Lea, Proc. Phys. Sot. (London) B76 (1960) 979. [3] K.D.Eowers, R.A.Kamper and C.D.Lustig. Proc. Phys. Sot. (London) B70 (1957) 1176. [4] ACarrington, D.&Levy and T.A.Miher; J. Chem. Phys. 45 (1966) 4093. (51 V.Jaccarino, J.G.King, R.A.Satten and H.H.Stroke, Phys. Rev. 94 (1954) 1798. [6]~M.S.d~~Groot,C.k deLangeandA.A.Monster, to be ‘published: :.
Volume 11, number 3
“FORBIDDEN” TRANSITIONS IN THE GAS PHASE PAIL&MAGNETICRESONANCE SPEClXA OF HALOGEN ATOMS
Received 27 August 197 1
Electron resonance gas phase transitions with a polarization paraUe1 to the externa! maSrwtic fietd are reported * in halogen atoms Atoms are produced inside the ESR cavity by means of a hot-wire method.
1. Introduction
Electron resonance spectra of chlorine f l] , bromine [Z] and iodine [31 atoms in their 2P,,2 ground state have been reported. The observed transitions were alI of the type A&fJ = t 1, AMI = 0 in the high-field quantum number representation. They were induced by the magnetic component of the microwave tieId perpendi~~l~ to the external magnetic field. Transitions arising from the pa&e1 magnetic component of the microwave field have only been detected so far in fluorine atoms [4] _ En ail cases the atoms were produced by a rnicrowave discharge in a gas stream entering the ESR cavity. We wish to report the detection of various “paraG transitions” in halogen atoms obtained by thermolysis.
2, Experimental Halogen atoms were produced by means of thermal dissociation inside a cylindrical, vacuum-tint cavity operating in the T&O1 1 mode at approximately 9180 MHz. A platinum w&e, fitted along the cavity axis and oriented parallel to the external magnetic field, could.be heated electrically. ~e~olysis of ‘chlbrine, bromine and io~~‘v~pour produced strong absorption signals due to halogen atoms. Linewidths were approximately 1,G. Because the atoms are produced in the r&on of the cavity axis, the ex~e~en~ a~~gement is optimized for the detection of $arailel trtisitjons. : . ..
-
‘.‘
3. Theory The spectra of halogen atoms in their 2T?,i2ground state can be described s&isf’ctoriIy by the following hamiltonian, working within the ground&ate manifold:
The three terms represent the Zeemart term sl,, the nudearelectron magnetic hyperfine coughing 5FD and the electric q~d~~~e ~terac~~R Q, respectively. IfA and/or e2@ are large, offdagonal elements ‘,
-. :
:..
.’ :
A Varizn V4500 spectrometer was used in conjunction with a 9 in magnet. The klystron frequency was measured with a Hewlett-Packard 52452, frequency counter in combination with a S25SA frequency converter. The magnetic field values were measured by means of an AEG nuclear magnetic resonance gaussmeter, in conjunction with the frequency counter men~oned above. The pressure in the vacuum system was appro~mately 0.1 torr. The cavity was connected through a liquid-nitrogen trap to a Precision Scientific Company two-stage vacuum pump with a capacity of f 500 ljmin. Low flow rates or no flaw at al.Iproved sufficient to obtain good spectra.
..
:.
,-
_’
._,
: 1
.
:
.‘.
.-,,,.
‘..
:
.._
.’ ,.,.,
._.‘I
.)
:
.‘.
‘,.,.
.
.
., volume ;-
!i, number ..
3
CHEMICAL PHYSICS LEl-iZRS
,’
become significant and mixing of the IZMI,JMJ) levels ,b+oties important. It appears that the terms I-I and (IsJ’)~ in the, hamiltonian give rise to the mixing of drfferent levels:1.7 connacts IIM ;JM;) with ILVI~l,J_M~ 1) levels, and (1.J) a couples luM,,.M,> -withboth IL%f$l,JMJ~l) and liM,+2, JMJT2)_ By .applying these operators, either separately or successively, three types of mixing which may induce transitions with parallei polarization can occur: (a).between IIMI,JMJ>and IIMI+ 1, JMp i ) levels 41J parallel transitions are possible; (b) between IIMI,JMJ)and ILWI+2, JMJT2) levels (U- l)(W-1) parallel transitions, occurring at ap proximately. “half field” are possible; (c) between IMI,JMJ) and ILUI+3,JMJT3) levels (2f-2)(U-2) parallel transitions, occurring at ap proximately “one third field” are possible.
4. Results A computer programme was written to diagonal&e the hamiltonian (,I.), Energy levels for different r-nag netic field values were calculated and transition fields
Tabie 1 Comparison of observed and calculated magnetic field values, and computed intensities, for “half- and third-field” transidons in rz71 atoms. The experimental error in line positions is * 1 G, the spectrometer frequency 9177 MHz Observed
Calculated
Relative intensity
3069.6 2456.1 2307.5 2045.7 1660.2 1852.4 1691.3 1428.3 1379.5 load 466.1
3070.9 2457.2 2308.5 2046.6 1861.1 1853.3 1693.5 1430.4 1381.1 967.6 687.7
4.8 9.1 5.9 4.6 7.2 3.6 1.8 2.7 2.8 0.2 1.0
:
,: ,,-
1379.5G
-H
Fig. 1. Part of the low-field q&rum of “‘1 atoms (spectiometer frequency 9177 MHz).
were obtained by linear interpolation. Relative intensities for the parallel transitions were also computed. For 12JI (nuclear spin S/2) out of 15 + 8 + 3 = 26 possible parallel transitions, 25 were detected, the
missing one being too weak to be observed. The iodine spectrum was calculated, using values for A and e2qQ obtained from atomic-beam work [5], and agJ value obtained from gas phase ESR [3] . Good agreement between calculated and observed line positions was found. Low-field results for iodine are given in table 1, and part of the low-field spectrum in fig. 1. For low magnetic fields, where the Zeeman term is no longer dominant, the mixing of energy levels becomes very extensive and therefore the quantum numbers MI and A/i, can no longer be used. Parallel transitions in chlorine, bromine and iodine at norr& magnetic field values and a complete description of the ESR cavity will be the subject of a separate publication [6].
References [l] V.Beluan-Lopez and H.G.Rob’inson, Phys. Rev. 123 (1961) 161. [2] J.S.M.Harvey, R.A.Kamper andK.R.Lea, Proc. Phys. Sot. (London) B76 (1960) 979. [3] K.D.Eowers, R.A.Kamper and C.D.Lustig. Proc. Phys. Sot. (London) B70 (1957) 1176. [4] ACarrington, D.&Levy and T.A.Miher; J. Chem. Phys. 45 (1966) 4093. (51 V.Jaccarino, J.G.King, R.A.Satten and H.H.Stroke, Phys. Rev. 94 (1954) 1798. [6]~M.S.d~~Groot,C.k deLangeandA.A.Monster, to be ‘published: :.