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Observation of emission from XeOH exciplexes
Volume 54. number 2
1 March 1978
CHEMICAL PHYSICS LET”I-ERS
OBSERVATION OF EMISSION FROM XeOH EXCIPLEXES M.H.R. HUTCHINSON Blackett Laboratory. Imperial College. London SW7 282. UK Received 28 November 1977
The tirst obscrvdtion of emission from a noble gas bydroxidc exciplex (XcOH) iq reported. Preliminary measurements indicate an emission band of wavelength A = 234 nm and 3 nm bandwidtil. l%e spontaneous decay time is <4 ns.
Following the investigation of the reactions of metastable noble gas atoms by Golde and Thrush [ l] and Velazco and Setser [2], the noble gas halide excirners have attracted considerable interest because they have made possible the development of efficient ultraviolet lasers. Emission spectra of practically all the halides of the heavy noble gases have been studied and laser action from many has been observed when excited by electron beams and by stabilized and unstabilized discharges. The formation of an ionic bend between an excited noble gas atom and a halogen ic a consequence of the strong similarity between the structure of an excited noble gas atom and an alkali metal in the ground state, both of which have a single eiectron crbiting a core of unit positive charge. Thus the metastable and rcsonancc states of the noble gases have ionization potentials which arc similar to those of the alkali metals which are adjacent to them in the periodic table and have similar chemical properties. By analogy with the reactions of the alkali metals, noble gas excimers and exciplexcs other than the halides might be expected to be formed by reactions between excited gas atoms and compounds which can act as donors of a suitable radical. In particular, ionic molccules may be formed with radicals with high electron affinitics. Of course, for the reaction to be energetically possible, the dissociation energy of the donor molecule must be less than that of the exciplex. Moreover, rapid and efficient exciplex formation requires both a large rate of reaction between the excited noble gas and the donor mclecule and the probability of processes such as predissociation and quenching to be small.
In a manner similar to the well known reactions of the alkali metals with water, noble gas hydroxide cxciplexes might be expected to be produced by the reaction Xe* + HZ0 + XcOIl*
+ H _
The spectroscopic properties of XeOH can be deduced by comparison with the analogous alkali hydroxide, CsOII. For sbch an ionic molecule, the energy of the electronically excited exciplcx is given by I(Xe) EA (OH) - E,(XeOH*) where I(Xe) is the ionization potential of xenon (12.13 ev), EA(OH) IS the electron affinity of the Oii radical (1.83 eV) [3] and Et(XeOH*) is the energ: required to dissociate the molecule into the ion pair, Xe+ + Oli-. It has been suggested [4] that, since the polar dissociation cncrgies of the halides [S] of a given alkali metal arc a monotonic function of the size of the I alogcn atom, the dissociation energies of other simple ionic aikali metal compounds (e.g. CsO [6] ) may bti calculated purely on the basis of size. Since the 011- io;t is intermediate in size between F- and Cl-, the polar dissociation energy of CsOIl is taken to have a value of 5.0 eV which is the average of the calculated values for CsF and CsCl [5]. This indicates that the energy of XcOH* would be -5.3 eV and that the wavelength of the radiation emitted in a transition to the ground sta!e which may be repulsive would be 2234 nm. This fails outside the absorption band of water, whereas a similar calculation for KrOH indicates 2 wavelength of 2 173 run. This radiation would be absorbed by water vspour, alrhough some other suitable OH donor might be found. 359
CHEBIICAL PHYSICS LETTERS
Volume 54, number 2
1 March 1978
Fig. 2. Temporal profile of the exciplex cmiscion when cvzltcd by 3 pulse of electrons of 4 ns duration. The scale is 10 ns per maJor scale division.
250
230
240
220
Abm) Flg. 1. Mlcrodensitomctcr XeOH.
tr.tce of the emission spectrum ot
To investigate the existence of XcOH* a mixture of Ar(2200 torr). Xe(80 torr) and water vapour (= 10 torr) was cxcitcd using it 30 kA beam of 600 kV electrons in a pulse of 35 ns duration (Physics International 1 lOA). The spectrum of the emission was recorded using a 600 mm spectrograph and Kodak SC7 ultraviolet sensitive film. This showed strong emission on the A “Z+-X ‘II band of OH at 306.4 run and a band of comparable intensity of 3 nm bandwidth centrcd at 234 nm. With a spectral resolution of *OS nm, no structure is observed and the emission is attributed to XeOH. A microdcnsitometcr trace of the spectrum is shown in fig. 1. Although the very good agrccmcnt of the wavelength of the observed emission with the minimum theoretical value is fortuitous, it does indicate that, like the noble gas halides [7], the ground state is at most only weakly repulsive for the interatomic distances at which emission occurs. It has yet to be established whether the OH emission arises from predissociation of the XeOH excip!ex or is due entirely to dissociation of the water vapour. To investigate the temporal behaviour of the emission, the gas mixture was excited by a 4 ns pulse of 600 keV electrons in a coaxial diode [8] . The emission was monitored with a solar blind CsTc photocathode (ITT 4115) for which the sensitivity at the wavelength
360
of the exciplex emission is approximately twenty times that at the wavelength of the OH emission (306.4 nm). The signal, measured on a Tektronix type 5 19 oscilloscope is shown in fig. 2 and indicates a decay time of the fiuorcsccncc of 54 ns. Although no attempt has so far been made to optirnise the partial pressures of the gases in the mixture to produce maximum fluorescence intensity, the intensity was foud to increase with increasing argon pressure up to the maximum studied, 4 500 torr, with no change in the observed fluorescence decay time. The total power of the cmittcd exciplex radiation is estimated to bc x IO6 W. In conclusion, preliminary investigations indicate the existence of emission from XeOH*. Further experiments are being carried out to explore the potential of such exciplexcs as laser systems. The provision of rcscarch facilities by Professor D.J. Bradley is gratefully acknowledged.
References 111 M-F. Golde and B.A. Thrush. Chcm. Pbys. Letters 29 (1974) 486.
I21 J-E. Velazco
and D-W. Setser. J. Chem. Phy~. 62(1975)
1990.
131 L.hf. Branscomb, Phys. Rev. 99A (1955) 1657. 141 L. Brewer and J. Margrave. J. Phys. Chcm. 59 (1955) 421. I51 Y.P. Varshni and R.C. Shukla, J. Mol. Spectry. 16 (1965)
63. 161 IANAF Thllermochcmical Tables, 2nd Ed.. NSRDS-NBS 37 (1971).
[7] J.J. Ewing and C.A. Brau, Phys. Rev. Al2 (1975) 129. [8l D.J. Bradley, D.R. Hull, M.H.R. Hutchinson and M.W. McGcoch, Opt. Commun. 14 (1975) 1.
1 March 1978
CHEMICAL PHYSICS LET”I-ERS
OBSERVATION OF EMISSION FROM XeOH EXCIPLEXES M.H.R. HUTCHINSON Blackett Laboratory. Imperial College. London SW7 282. UK Received 28 November 1977
The tirst obscrvdtion of emission from a noble gas bydroxidc exciplex (XcOH) iq reported. Preliminary measurements indicate an emission band of wavelength A = 234 nm and 3 nm bandwidtil. l%e spontaneous decay time is <4 ns.
Following the investigation of the reactions of metastable noble gas atoms by Golde and Thrush [ l] and Velazco and Setser [2], the noble gas halide excirners have attracted considerable interest because they have made possible the development of efficient ultraviolet lasers. Emission spectra of practically all the halides of the heavy noble gases have been studied and laser action from many has been observed when excited by electron beams and by stabilized and unstabilized discharges. The formation of an ionic bend between an excited noble gas atom and a halogen ic a consequence of the strong similarity between the structure of an excited noble gas atom and an alkali metal in the ground state, both of which have a single eiectron crbiting a core of unit positive charge. Thus the metastable and rcsonancc states of the noble gases have ionization potentials which arc similar to those of the alkali metals which are adjacent to them in the periodic table and have similar chemical properties. By analogy with the reactions of the alkali metals, noble gas excimers and exciplexcs other than the halides might be expected to be formed by reactions between excited gas atoms and compounds which can act as donors of a suitable radical. In particular, ionic molccules may be formed with radicals with high electron affinitics. Of course, for the reaction to be energetically possible, the dissociation energy of the donor molecule must be less than that of the exciplex. Moreover, rapid and efficient exciplex formation requires both a large rate of reaction between the excited noble gas and the donor mclecule and the probability of processes such as predissociation and quenching to be small.
In a manner similar to the well known reactions of the alkali metals with water, noble gas hydroxide cxciplexes might be expected to be produced by the reaction Xe* + HZ0 + XcOIl*
+ H _
The spectroscopic properties of XeOH can be deduced by comparison with the analogous alkali hydroxide, CsOII. For sbch an ionic molecule, the energy of the electronically excited exciplcx is given by I(Xe) EA (OH) - E,(XeOH*) where I(Xe) is the ionization potential of xenon (12.13 ev), EA(OH) IS the electron affinity of the Oii radical (1.83 eV) [3] and Et(XeOH*) is the energ: required to dissociate the molecule into the ion pair, Xe+ + Oli-. It has been suggested [4] that, since the polar dissociation cncrgies of the halides [S] of a given alkali metal arc a monotonic function of the size of the I alogcn atom, the dissociation energies of other simple ionic aikali metal compounds (e.g. CsO [6] ) may bti calculated purely on the basis of size. Since the 011- io;t is intermediate in size between F- and Cl-, the polar dissociation energy of CsOIl is taken to have a value of 5.0 eV which is the average of the calculated values for CsF and CsCl [5]. This indicates that the energy of XcOH* would be -5.3 eV and that the wavelength of the radiation emitted in a transition to the ground sta!e which may be repulsive would be 2234 nm. This fails outside the absorption band of water, whereas a similar calculation for KrOH indicates 2 wavelength of 2 173 run. This radiation would be absorbed by water vspour, alrhough some other suitable OH donor might be found. 359
CHEBIICAL PHYSICS LETTERS
Volume 54, number 2
1 March 1978
Fig. 2. Temporal profile of the exciplex cmiscion when cvzltcd by 3 pulse of electrons of 4 ns duration. The scale is 10 ns per maJor scale division.
250
230
240
220
Abm) Flg. 1. Mlcrodensitomctcr XeOH.
tr.tce of the emission spectrum ot
To investigate the existence of XcOH* a mixture of Ar(2200 torr). Xe(80 torr) and water vapour (= 10 torr) was cxcitcd using it 30 kA beam of 600 kV electrons in a pulse of 35 ns duration (Physics International 1 lOA). The spectrum of the emission was recorded using a 600 mm spectrograph and Kodak SC7 ultraviolet sensitive film. This showed strong emission on the A “Z+-X ‘II band of OH at 306.4 run and a band of comparable intensity of 3 nm bandwidth centrcd at 234 nm. With a spectral resolution of *OS nm, no structure is observed and the emission is attributed to XeOH. A microdcnsitometcr trace of the spectrum is shown in fig. 1. Although the very good agrccmcnt of the wavelength of the observed emission with the minimum theoretical value is fortuitous, it does indicate that, like the noble gas halides [7], the ground state is at most only weakly repulsive for the interatomic distances at which emission occurs. It has yet to be established whether the OH emission arises from predissociation of the XeOH excip!ex or is due entirely to dissociation of the water vapour. To investigate the temporal behaviour of the emission, the gas mixture was excited by a 4 ns pulse of 600 keV electrons in a coaxial diode [8] . The emission was monitored with a solar blind CsTc photocathode (ITT 4115) for which the sensitivity at the wavelength
360
of the exciplex emission is approximately twenty times that at the wavelength of the OH emission (306.4 nm). The signal, measured on a Tektronix type 5 19 oscilloscope is shown in fig. 2 and indicates a decay time of the fiuorcsccncc of 54 ns. Although no attempt has so far been made to optirnise the partial pressures of the gases in the mixture to produce maximum fluorescence intensity, the intensity was foud to increase with increasing argon pressure up to the maximum studied, 4 500 torr, with no change in the observed fluorescence decay time. The total power of the cmittcd exciplex radiation is estimated to bc x IO6 W. In conclusion, preliminary investigations indicate the existence of emission from XeOH*. Further experiments are being carried out to explore the potential of such exciplexcs as laser systems. The provision of rcscarch facilities by Professor D.J. Bradley is gratefully acknowledged.
References 111 M-F. Golde and B.A. Thrush. Chcm. Pbys. Letters 29 (1974) 486.
I21 J-E. Velazco
and D-W. Setser. J. Chem. Phy~. 62(1975)
1990.
131 L.hf. Branscomb, Phys. Rev. 99A (1955) 1657. 141 L. Brewer and J. Margrave. J. Phys. Chcm. 59 (1955) 421. I51 Y.P. Varshni and R.C. Shukla, J. Mol. Spectry. 16 (1965)
63. 161 IANAF Thllermochcmical Tables, 2nd Ed.. NSRDS-NBS 37 (1971).
[7] J.J. Ewing and C.A. Brau, Phys. Rev. Al2 (1975) 129. [8l D.J. Bradley, D.R. Hull, M.H.R. Hutchinson and M.W. McGcoch, Opt. Commun. 14 (1975) 1.