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Penetration depths of photomobilized F atoms from a sandwich experiment
JOURNAL OF
LUMINESCENCE ELSEVIER
Journal
of Luminescence
72-74
(1997) 912-913
Penetration depths of photomobilized F atoms from a sandwich experiment M. Dickgiesser *, C. Bressler, N. Schwentner Institut fir Experimentalphysik, Freie Universitiit Berlin, Arnimallee 14, D-14195 Berlin, Germany
Abstract Photodissociation of F, in Ar at 10 K with 10.15 eV leads to mobile F atoms with a kinetic energy of 4.2 eV. The F atoms are sent through an Ar spacer layer with thickness varying from 0 nm up to 10 nm and atoms arriving at the interface to a Kr substrate are detected by means of Kr,F luminescence. The intensity represents the amount of F atoms which have penetrated the spacer layer and depends strongly on the layer thickness. We found an exponential distribution of penetration depths with a mean value of 2.5 nm corresponding to seven nearest-neighbour distances. This shows that the migration of F in Ar is characterized by an exceptionally high mobility in contrast to the other halide
atoms. Keywords:
Photodissociation;
Migration; Matrix isolation spectroscopy
1. Experiment We prepare sandwiches composed of a detection layer of 38 nm Kr, an intermediate Ar spacer layer (O-10 nm) and a toplayer of 5 nm Ar/F2 (with an Fz concentration of 0.5%) on a cooled MgF,-substrate under UHV conditions. The layer thicknesses are controlled by a simultaneously covered quartz microbalance with an accuracy of 0.2 nm. The samples are irradiated with monochromatic synchrotron radiation of 10.15 eV during deposition which guarantees an efficient dissociation of the Fz molecules on a repulsive potential surface [l] producing hot F atoms with kinetic energies of
*Corresponding author. Tel.: + 49 30 8386084; fax: + 49 30 8383050; e-mail: [email protected].
0022-2313/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PIZ SOO22-23 13(96)00206-2
4.2 eV. Fig. 1 shows one of the growth curves of KrzF emission at 444 nm. It can be clearly seen that the Kr,F intensity increases strongly compared to the background with the beginning of the Ar/F, deposition and irradiation and finally reaches a saturation value. This value is proportional to the number of F atoms that have been transferred to the Ar/Kr interface and yields for the chosen Ar layer thickness d one point in the plot in Fig. 2. Fig. 2 shows the main result of our measurements: the saturation intensity decreases with increasing d and can be described quite well by an exponential with a mean value of 2.5 nm (solid line in Fig. 2) which represents the mean penetration depth. The maximum of the KrzF fluorescence band observed at 444 nm lies between the values for pure Ar (439 nm) and pure Kr (453 nm) which illustrates that the detected F atoms are located right at the interface.
M. Dickgiesser et al. /Journal of Luminescence 72-74 (1997) 912-913
913
is excluded by the weak absorption of F atoms in Ar at 10.15 eV. To confirm that the Ar films are uniform in thickness and do not contain holes we assure that the Kr substrate surface excitons are quenched for the chosen preparation conditions as described in Ref. [4]. The saturation yields and the penetration depths did not depend on the temperature during dissociation in the range from 4 to 12 K.
satlnatian
intensity
Li "0
5 10 Irradiation dose / 10’7photons / cm’
15
Fig. 1. Dependence of the Kr,F intensity at 444 nm on the irradiation dose during the deposition of a sample, the exciting photon energy is 10.15 eV.
Ar layer thickness d / mn
Fig. 2. Dependence of the Kr,F saturation intensity on the intermediate Ar layer thickness d yielding an exponential decay with a mean value of 2.5 nm (solid line).
The occurrence of a saturation plateau in Fig. 1 indicates that the F atoms remain at the Ar/Kr interface which is in accordance with the low kinetic energy of an F atom after radiative relaxation of Kr,F to the ground state [2] but is different to the photomobility discussed in Ref. [2]. The chosen photon energy of 10.15 eV corresponds to the energy of the Kr * (n = 1) excitons which efficiently transfer energy to the interface and to the trapped F atoms resulting in a large Kr,F fluorescence signal. An increased photomobility [3] which would lead to an overestimated penetration depth
2. Discussion The mean penetration depth of 2.5 nm represents an average over the angular distribution of the F atoms momentums after dissociation. Assuming a rectilinear migration of the F atoms as suggested by Ref. [S] and an isotropic angular distribution results in a value of 3 nm for the mean distance traveled by the F atoms. The large migration range of about 2.5 nm of F in Ar is in rough accordance with an early experimental study [3] where a value of about 7 nm was estimated for F atoms from the decay of Ar,F, but the 7 nm seem to be overestimated since the excess energy was only 1.7 eV. A calculation for Ar,F decay predicts at best cage exit but no further migration [6]. Another simulation for Fz dissociation with an excess energy of 2.0 eV at T = 4 K supports our experimental result by one long trajectory of 3 nm out of 50 calculated ones [S]. The absence of a temperature effect in the saturation yields is consistent with a calculated dissociation quantum yield of almost unity for T = 4 K and T = 12 K at high excess energies around 2.8 eV [S].
References [l] D.C. Cartwright and P.J. Hay, Chem. Phys. 114 (1987) 305. [Z] H. Kunttu, J. Feld, R. Alimi, A. Becker and V.A. Apkarian, J. Chem. Phys. 92 (1990) 4856. [3] J. Feld, H. Kunttu and V.A. Apkarian, J. Chem. Phys. 93 (1990) 1009. [4] C. Bressler, PhD thesis, ch. 5, Freie Universitiit Berlin (1995). [S] R. Alimi, R.B. Gerber and V.A. Apkarian, J. Chem. Phys. 92 (1990) 3551. [6] I.H. Gersonde and H. Gabriel, J. Chem. Phys. 98 (1993) 2094.
LUMINESCENCE ELSEVIER
Journal
of Luminescence
72-74
(1997) 912-913
Penetration depths of photomobilized F atoms from a sandwich experiment M. Dickgiesser *, C. Bressler, N. Schwentner Institut fir Experimentalphysik, Freie Universitiit Berlin, Arnimallee 14, D-14195 Berlin, Germany
Abstract Photodissociation of F, in Ar at 10 K with 10.15 eV leads to mobile F atoms with a kinetic energy of 4.2 eV. The F atoms are sent through an Ar spacer layer with thickness varying from 0 nm up to 10 nm and atoms arriving at the interface to a Kr substrate are detected by means of Kr,F luminescence. The intensity represents the amount of F atoms which have penetrated the spacer layer and depends strongly on the layer thickness. We found an exponential distribution of penetration depths with a mean value of 2.5 nm corresponding to seven nearest-neighbour distances. This shows that the migration of F in Ar is characterized by an exceptionally high mobility in contrast to the other halide
atoms. Keywords:
Photodissociation;
Migration; Matrix isolation spectroscopy
1. Experiment We prepare sandwiches composed of a detection layer of 38 nm Kr, an intermediate Ar spacer layer (O-10 nm) and a toplayer of 5 nm Ar/F2 (with an Fz concentration of 0.5%) on a cooled MgF,-substrate under UHV conditions. The layer thicknesses are controlled by a simultaneously covered quartz microbalance with an accuracy of 0.2 nm. The samples are irradiated with monochromatic synchrotron radiation of 10.15 eV during deposition which guarantees an efficient dissociation of the Fz molecules on a repulsive potential surface [l] producing hot F atoms with kinetic energies of
*Corresponding author. Tel.: + 49 30 8386084; fax: + 49 30 8383050; e-mail: [email protected].
0022-2313/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PIZ SOO22-23 13(96)00206-2
4.2 eV. Fig. 1 shows one of the growth curves of KrzF emission at 444 nm. It can be clearly seen that the Kr,F intensity increases strongly compared to the background with the beginning of the Ar/F, deposition and irradiation and finally reaches a saturation value. This value is proportional to the number of F atoms that have been transferred to the Ar/Kr interface and yields for the chosen Ar layer thickness d one point in the plot in Fig. 2. Fig. 2 shows the main result of our measurements: the saturation intensity decreases with increasing d and can be described quite well by an exponential with a mean value of 2.5 nm (solid line in Fig. 2) which represents the mean penetration depth. The maximum of the KrzF fluorescence band observed at 444 nm lies between the values for pure Ar (439 nm) and pure Kr (453 nm) which illustrates that the detected F atoms are located right at the interface.
M. Dickgiesser et al. /Journal of Luminescence 72-74 (1997) 912-913
913
is excluded by the weak absorption of F atoms in Ar at 10.15 eV. To confirm that the Ar films are uniform in thickness and do not contain holes we assure that the Kr substrate surface excitons are quenched for the chosen preparation conditions as described in Ref. [4]. The saturation yields and the penetration depths did not depend on the temperature during dissociation in the range from 4 to 12 K.
satlnatian
intensity
Li "0
5 10 Irradiation dose / 10’7photons / cm’
15
Fig. 1. Dependence of the Kr,F intensity at 444 nm on the irradiation dose during the deposition of a sample, the exciting photon energy is 10.15 eV.
Ar layer thickness d / mn
Fig. 2. Dependence of the Kr,F saturation intensity on the intermediate Ar layer thickness d yielding an exponential decay with a mean value of 2.5 nm (solid line).
The occurrence of a saturation plateau in Fig. 1 indicates that the F atoms remain at the Ar/Kr interface which is in accordance with the low kinetic energy of an F atom after radiative relaxation of Kr,F to the ground state [2] but is different to the photomobility discussed in Ref. [2]. The chosen photon energy of 10.15 eV corresponds to the energy of the Kr * (n = 1) excitons which efficiently transfer energy to the interface and to the trapped F atoms resulting in a large Kr,F fluorescence signal. An increased photomobility [3] which would lead to an overestimated penetration depth
2. Discussion The mean penetration depth of 2.5 nm represents an average over the angular distribution of the F atoms momentums after dissociation. Assuming a rectilinear migration of the F atoms as suggested by Ref. [S] and an isotropic angular distribution results in a value of 3 nm for the mean distance traveled by the F atoms. The large migration range of about 2.5 nm of F in Ar is in rough accordance with an early experimental study [3] where a value of about 7 nm was estimated for F atoms from the decay of Ar,F, but the 7 nm seem to be overestimated since the excess energy was only 1.7 eV. A calculation for Ar,F decay predicts at best cage exit but no further migration [6]. Another simulation for Fz dissociation with an excess energy of 2.0 eV at T = 4 K supports our experimental result by one long trajectory of 3 nm out of 50 calculated ones [S]. The absence of a temperature effect in the saturation yields is consistent with a calculated dissociation quantum yield of almost unity for T = 4 K and T = 12 K at high excess energies around 2.8 eV [S].
References [l] D.C. Cartwright and P.J. Hay, Chem. Phys. 114 (1987) 305. [Z] H. Kunttu, J. Feld, R. Alimi, A. Becker and V.A. Apkarian, J. Chem. Phys. 92 (1990) 4856. [3] J. Feld, H. Kunttu and V.A. Apkarian, J. Chem. Phys. 93 (1990) 1009. [4] C. Bressler, PhD thesis, ch. 5, Freie Universitiit Berlin (1995). [S] R. Alimi, R.B. Gerber and V.A. Apkarian, J. Chem. Phys. 92 (1990) 3551. [6] I.H. Gersonde and H. Gabriel, J. Chem. Phys. 98 (1993) 2094.