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Deactivation of Hg(63P0)
Volume 6, number 5
CHEMICAL PHYSICSLETTERS
DEACTIVATION
OF
1 September
1970
Hg(S3P,,)
A. B. CALLRAH and J. C. McGUHK Physical
Chemistry
Laboratoy,
Cambridge,
Lensfield
Road.
UK
Received 9 June 1970
The excitation of Hg vapour/N
mixtures by a black-body Hash was shownto produce N
%c by doubk
pumping. The decay of Hg(6SPO)IollowLngthe flash therefore is not characteristic of the zA eactivation rate, since Hg(63PO)is still produced after the Cash by energy transfer from N2ASEt. It is clafmed that excita tion of Hg vapour/N2 mixtures with a new flash device wae effectively uncomplicated; the derived crosssections for deactivation of Hg(63PO)were foundto be much greater than previous estimates with the standard flash technique.
Hates of deactivation of Hg(63P6) (Hg ) were first reported by Callear and Norrish [lf Mercury vapour was excited with a standard, ‘black-body’ flash, Hg(6%6) + Av(2537A) - Hg(S3Pl) and spin-orbit gen,
relaxation was induced with nitro-
Hg(63Pl) + N2 4 Hg(S3P,) + N2 .
The decay of HgO with time was monitored by kinetic absorption spectroscopy in Hg/N2 mixtures, with small partial pressures of various added gases, Approximate rate coefficients for deactivation of HgO were derived. Later, with a similar experimental technique, CaLlear and Williams [2] published supposedly accurate cross sections for deactivation of IigO. We have recently described a new type of flash iamp [3] (C.M. lamp) which emits discrete atomic lines and negligible continuum. The discharge occurs perpendicularly to the lamp axis between 22 electrode pairs, and the reaction vessel of 5 mm diameter is situated between the electrodes, along the axis of the flash lamp. With the new technique applied to mercury photosensitized reactions, Qnomalously fast’ rates of Hgo decay were noted, with traces of H2, NG or CO2 added to Hg vapour in’an excess of N2., For example, with [Hg] = 2 X 10-3 torr + [N2] = 400 torr + [HZ] = 0.01 torr, the concentration of HgH reached approximately one third of the maximum attained in ti high pressure of H2 after a ttme delay of ~~20 psec. According to the_ reported
ai ice
No delay.
30’
16p rec.
42
31
54
46
:
66
61
82
89
--
.,
,.‘(a~
:
(b)
Fig. 1. Decay of Hg@PO) in 0.1 torr H2, 2 x 10-S torr Hg, 700 torr N2. (a) Black-body excitation. (h) MOnochromaticexcitation_
cross
section for deactivation of Ergo by HZ, only 0.5% of the Hg6 should react with 0.01 torr of H2 after 20 j&sec [Z]. A comparison of the Hgo decay rates with the two techniques is shown
in fig. 1. Pollowing a-detailed investigation of both types of experiment we have concluded that the experiment wfth the black-body flash is complicat& by the pr,oduction of N2A3Ct (Ni). This was estaI
.
4047 i
4047A
417
Y$me
6, nu@er
1 September 1970
CBXMICAL PHYSICS LETTERS
5
Table 1
Cross sections for deactivation of Hgo at’29&
.,
CalIear and William5
Present work
Ratecoefficient (cm3molecule’l set-1) 2.5 x 1orlO
NO
cross
Cross section
section
(cm2 x 1016)
(cm2 x 1016)
0.34
16.2
CO2
4-4 x 10’13
0.033
0.0014
Hz
5.4 x 10’11
0.95
0.018
blished by adding ~10-3 torr of NO to an Hg vapour/N2 mixture and observing the NOy emiesion, following ‘tie flash, with a photomultiplier. Direct excitation of the NO was avoided by use of a light filter. The dependence af the decay constant of the Y emission on the [NO] and [Hg] was in excellent agreement with published rate coefficients for enerw transfer from N-g to NO and Hg [4]. The. experiment with the black-body flash ts surprisingly complex and involves double pumping %ndlong energy chains, viz. Hg(6%,)
+ hv(2537& - Hg(63P1)
Hg(63PI) + N2 - Hg(B3Po) + N2 01
f4g(63Po) + hv 4 Hg** Hg** + N2 -* Hg(G%O) + N; N*+Hg(G.S 1
2
8
)‘N
2 c Hg(S3PJ)
-etc.
** may y any of the upper states of the =3he n S1Hg orn3D1-6 PO Rydberg series. Because of the ‘sloti’ formation of Hg after terminaticn of the flash, the recorded cross-sections for deactivation of Hgo by that niethod are much too small. With the CM.
flash,
the experiment is effec-
tively uncomplicated; N$ probably is produced by double pumping at 404’7~ (r3S1 - 63P~) though removal of the line with a bromine filter does not ,affect the [Hg ] time profiles. Rates of deactivation of Hgo &r H2, NO and CO2 have been determined by their addition, at various partial pressures, to Hg .vapour/Nz mixtures. The [Hgo] was monitored by kinetic absorption spectroscopy of the 404713line and also by a photoelectric technique. The reciprocal relaxation times were linearly dependent on [H2], (NO] or [CO2], ;as shown in fig. 2. The rate coefficients for deactivation of Hgg are givan by the slopes of the fig. 2 plots, and are listed in table i. We ~3 proceeding with the remeasurement of the deactivation cross sections of all the con_ pounds which were studied by Callear and Wil.. ’ 418
I
O‘S
I
I
l.0
15
1
20
Concmtration of CO, ftorrl. Hz ttorrxlOz). No lhxr=ldL Fig. 2. Linear dependence of reciprocal relaxation time on partial pressure of added gases, with 2 x 10’3 torr Hg. 700 torr N2 A. Hz;Cl, CO2; 0, NO.
Urns. It is remarkable how earlier workers were deceived by what appeared to be a very simple experiment.
The quantum yields for for-
mation of Hgo, reported recently by Callear and and Hedges [5], must now be considered to be lower lipits because of substantial loss of Hgg at the time of observation. REFERENCES [l] A. B. Callear and R. G. W. Norrish. Proc. Roy. Sot. A 266 (1962) 299. [2JA. B. Callear and G. J. Williams, Trans. Faraday Sot. 60 (1964) 2158. [3] ;$3. Callear and J. C. MdGurk, Nature 266 (1970) [4] A. 6. Callear and P.M. Wood, Chem. Phys. Letters 5 (1970) 128. [S] A. B.CaUear ax@ 3 E.M.Hedges. Trans. Faraday Sot. 66 (1970) 605.
CHEMICAL PHYSICSLETTERS
DEACTIVATION
OF
1 September
1970
Hg(S3P,,)
A. B. CALLRAH and J. C. McGUHK Physical
Chemistry
Laboratoy,
Cambridge,
Lensfield
Road.
UK
Received 9 June 1970
The excitation of Hg vapour/N
mixtures by a black-body Hash was shownto produce N
%c by doubk
pumping. The decay of Hg(6SPO)IollowLngthe flash therefore is not characteristic of the zA eactivation rate, since Hg(63PO)is still produced after the Cash by energy transfer from N2ASEt. It is clafmed that excita tion of Hg vapour/N2 mixtures with a new flash device wae effectively uncomplicated; the derived crosssections for deactivation of Hg(63PO)were foundto be much greater than previous estimates with the standard flash technique.
Hates of deactivation of Hg(63P6) (Hg ) were first reported by Callear and Norrish [lf Mercury vapour was excited with a standard, ‘black-body’ flash, Hg(6%6) + Av(2537A) - Hg(S3Pl) and spin-orbit gen,
relaxation was induced with nitro-
Hg(63Pl) + N2 4 Hg(S3P,) + N2 .
The decay of HgO with time was monitored by kinetic absorption spectroscopy in Hg/N2 mixtures, with small partial pressures of various added gases, Approximate rate coefficients for deactivation of HgO were derived. Later, with a similar experimental technique, CaLlear and Williams [2] published supposedly accurate cross sections for deactivation of IigO. We have recently described a new type of flash iamp [3] (C.M. lamp) which emits discrete atomic lines and negligible continuum. The discharge occurs perpendicularly to the lamp axis between 22 electrode pairs, and the reaction vessel of 5 mm diameter is situated between the electrodes, along the axis of the flash lamp. With the new technique applied to mercury photosensitized reactions, Qnomalously fast’ rates of Hgo decay were noted, with traces of H2, NG or CO2 added to Hg vapour in’an excess of N2., For example, with [Hg] = 2 X 10-3 torr + [N2] = 400 torr + [HZ] = 0.01 torr, the concentration of HgH reached approximately one third of the maximum attained in ti high pressure of H2 after a ttme delay of ~~20 psec. According to the_ reported
ai ice
No delay.
30’
16p rec.
42
31
54
46
:
66
61
82
89
--
.,
,.‘(a~
:
(b)
Fig. 1. Decay of Hg@PO) in 0.1 torr H2, 2 x 10-S torr Hg, 700 torr N2. (a) Black-body excitation. (h) MOnochromaticexcitation_
cross
section for deactivation of Ergo by HZ, only 0.5% of the Hg6 should react with 0.01 torr of H2 after 20 j&sec [Z]. A comparison of the Hgo decay rates with the two techniques is shown
in fig. 1. Pollowing a-detailed investigation of both types of experiment we have concluded that the experiment wfth the black-body flash is complicat& by the pr,oduction of N2A3Ct (Ni). This was estaI
.
4047 i
4047A
417
Y$me
6, nu@er
1 September 1970
CBXMICAL PHYSICS LETTERS
5
Table 1
Cross sections for deactivation of Hgo at’29&
.,
CalIear and William5
Present work
Ratecoefficient (cm3molecule’l set-1) 2.5 x 1orlO
NO
cross
Cross section
section
(cm2 x 1016)
(cm2 x 1016)
0.34
16.2
CO2
4-4 x 10’13
0.033
0.0014
Hz
5.4 x 10’11
0.95
0.018
blished by adding ~10-3 torr of NO to an Hg vapour/N2 mixture and observing the NOy emiesion, following ‘tie flash, with a photomultiplier. Direct excitation of the NO was avoided by use of a light filter. The dependence af the decay constant of the Y emission on the [NO] and [Hg] was in excellent agreement with published rate coefficients for enerw transfer from N-g to NO and Hg [4]. The. experiment with the black-body flash ts surprisingly complex and involves double pumping %ndlong energy chains, viz. Hg(6%,)
+ hv(2537& - Hg(63P1)
Hg(63PI) + N2 - Hg(B3Po) + N2 01
f4g(63Po) + hv 4 Hg** Hg** + N2 -* Hg(G%O) + N; N*+Hg(G.S 1
2
8
)‘N
2 c Hg(S3PJ)
-etc.
** may y any of the upper states of the =3he n S1Hg orn3D1-6 PO Rydberg series. Because of the ‘sloti’ formation of Hg after terminaticn of the flash, the recorded cross-sections for deactivation of Hgo by that niethod are much too small. With the CM.
flash,
the experiment is effec-
tively uncomplicated; N$ probably is produced by double pumping at 404’7~ (r3S1 - 63P~) though removal of the line with a bromine filter does not ,affect the [Hg ] time profiles. Rates of deactivation of Hgo &r H2, NO and CO2 have been determined by their addition, at various partial pressures, to Hg .vapour/Nz mixtures. The [Hgo] was monitored by kinetic absorption spectroscopy of the 404713line and also by a photoelectric technique. The reciprocal relaxation times were linearly dependent on [H2], (NO] or [CO2], ;as shown in fig. 2. The rate coefficients for deactivation of Hgg are givan by the slopes of the fig. 2 plots, and are listed in table i. We ~3 proceeding with the remeasurement of the deactivation cross sections of all the con_ pounds which were studied by Callear and Wil.. ’ 418
I
O‘S
I
I
l.0
15
1
20
Concmtration of CO, ftorrl. Hz ttorrxlOz). No lhxr=ldL Fig. 2. Linear dependence of reciprocal relaxation time on partial pressure of added gases, with 2 x 10’3 torr Hg. 700 torr N2 A. Hz;Cl, CO2; 0, NO.
Urns. It is remarkable how earlier workers were deceived by what appeared to be a very simple experiment.
The quantum yields for for-
mation of Hgo, reported recently by Callear and and Hedges [5], must now be considered to be lower lipits because of substantial loss of Hgg at the time of observation. REFERENCES [l] A. B. Callear and R. G. W. Norrish. Proc. Roy. Sot. A 266 (1962) 299. [2JA. B. Callear and G. J. Williams, Trans. Faraday Sot. 60 (1964) 2158. [3] ;$3. Callear and J. C. MdGurk, Nature 266 (1970) [4] A. 6. Callear and P.M. Wood, Chem. Phys. Letters 5 (1970) 128. [S] A. B.CaUear ax@ 3 E.M.Hedges. Trans. Faraday Sot. 66 (1970) 605.