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Polarization effects in laser photofragmentation of Rydberg Matter clusters KN∗ in a weak electric field
21 July 2000
Chemical Physics Letters 325 Ž2000. 264–268 www.elsevier.nlrlocatercplett
Polarization effects in laser photofragmentation of Rydberg Matter clusters K )N in a weak electric field Jiaxi Wang 1, Leif Holmlid ) Reaction Dynamics Group, Department of Chemistry, Goteborg UniÕersity, SE-412 96 Goteborg, Sweden ¨ ¨ Received 4 January 2000; in final form 20 March 2000
Abstract The kinetic energy release and its variation with laser polarization in laser fragmentation of Rydberg Matter clusters is studied with a power density - 10 9 W cmy2 . The fragment ions observed are Kq N with N s 1, 2, 3, 4 and 7. The excess kinetic energy of the fragment ions observed with the laser polarization in the detector plane is approximately 0.4 eV. The fragmentation is sensitive to the external field strength at 1–3 V cmy1. The interionic distance in the Coulomb explosions is 4 nm. The polarization effect for ns pulses indicates slow cluster rotation and a cluster size of ) 0.3 mm. q 2000 Published by Elsevier Science B.V.
1. Introduction We have recently developed a laser-based method to investigate the formation of Rydberg and Rydberg Matter clusters at solid surfaces w1x. We now apply it to the study of polarization effects in the laser-induced fragmentation of small Rydberg Matter clusters, K N , formed outside a graphite basal surface. The ion time-of-flight ŽTOF. spectra vary clearly with the polarization direction of the laser, with a well separated fast ion peak when the laser polarization is in the same plane as the detector. Such effects are usually understood as a fragmentation induced by field ionization of the cluster, during which process
) Corresponding author: Fax: q46-31 7723107; e-mail [email protected] 1 Present address. Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ont. M5S 1A1, Canada.
two or more electrons are stripped off the cluster or molecule. Following the field ionization, so-called Coulomb explosions take place when the bonding in the cluster or molecule is replaced by the repulsion between two positive ion charges. The use of lasers with pulse lengths of 100 fs or less has in the last few years made it possible to fragment ground state molecules by such electron stripping giving Coulomb explosions. For example, the Coulomb explosions of small molecules like I 2 , N2 and H 2 were studied by Posthumus et al. w2–4x and by Banerjee et al. w5x. In the case of N2 and H 2 it was concluded that the molecules were forced into so-called dynamic alignment by the laser field. Several groups also study similar processes in clusters. Na clusters have been studied by Schapiro et al. w6x, and a microcanonical description was used to interpret the results in terms of phase transitions in the clusters. For a review of cluster ionization and fragmentation, see Ref. w7x.
0009-2614r00r$ - see front matter q 2000 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 6 4 0 - 0
J. Wang, L. Holmlidr Chemical Physics Letters 325 (2000) 264–268
2. Experimental The experiments are carried out in an UHV apparatus with a base pressure of 1 = 10y8 mbar. This has been fully described previously w8,9x. The pyrolytic graphite sample in the center of the UHV chamber was cut from a crystal ŽGrade ZYB, Advanced Ceramics Corp.. as a rod with a thickness of 0.8 mm, a width of 3.3 mm, and a length of 30 mm. It is mounted in the vertical direction, clamped at the ends by Ta foils, exposing a length of 16 mm of its basal Ž0001. surface. The sample is at room temperature in the laser experiments. The sample is filled with K from a molecular beam as described below, usually the day before the TOF experiments. As shown in Ref. w10x, the desorption and emission of K from a graphite surface gives K ) Rydberg states. These Rydberg states condense to form a cloud of Rydberg Matter outside the sample, from which well-defined clusters can be released by laser pulses w1x. A Nd:YAG laser with a maximum average power of 1 W at 532 nm wavelength and 7 ns pulses with a 10 Hz repetition rate is used to pump the dye laser ŽLambda-Physik.. Rhodamine 6G is used to give radiation at 564.0 nm in 5 ns pulses at a maximum average power at the UHV chamber of 100 mW. The laser beam diameter is approximately 3 mm without focusing, and the corresponding power density is less than 1.5 W cmy2 . A single lens with a focal length of 0.6 m is used to focus the laser beam. With focusing, the peak power density in the pulse is less than 10 9 W cmy2 , corresponding to a beam waist diameter of 0.4 mm. The laser beam enters through a window in the vacuum chamber and passes in front of the graphite sample at a distance of 5–15 mm. Its plane of polarization can be changed by rotating a half-wave plate in the laser beam, positioned close to the vacuum window to avoid any unwanted motion of the laser beam during the change of polarization direction. The final dichroic mirror after the halfwave plate does not change its reflectance appreciably for the two polarization directions. The detector box is placed 8.5 cm in front of the sample. The voltages on the sample are 2.5–20 V, corresponding to approximately 0.5–4 Vrcm at the laser beam position. The only particles which can be measured by the detector are positive ions, and Rydberg states
265
which field ionize in the detector to give positive ions. The ions are drawn down into a channel electron multiplier where they are amplified. The resulting pulses are further amplified, brought through a discriminator and collected in a multi-channel analyzer. Usually, 500 periods of the signal are averaged and output to a computer. To fill the graphite sample with K atoms, a K molecular beam is directed towards the graphite sample at an angle of 458 relative to the normal of the exposed basal surface, while the voltage of the sample is y10 V and its temperature is raised to 1500 K by passing an AC current of approximately 30 A through it. The K beam is a thermal beam from a two-chamber source, with the reservoir at approximately 525 K and the front of the source 50–100 K hotter. The source chamber is separately pumped, with one further intermediate differential stage to decrease the gas load to the UHV sample chamber. The impinging beam flux density is of the order of 5 = 10y8 A cmy2 or 3 = 10 11 atoms cmy2 sy1 at the sample, corresponding to 1 = 10y9 mbar pressure. At this flux density, a monolayer would be formed in 1 h if no desorption or diffusion into the sample took place. If only desorption took place, the surface density of K would be 10y1 0 of a monolayer, using rates of desorption from Ref. w10x. However, the diffusion into the bulk is much faster than the desorption w11x. The time spent for each K atom on the surface before diffusing into the bulk is estimated to be - 100 m s. A beam flag in front of the source and a valve between the source chamber and the sample chamber can be used to turn off the beam. During the laser experiments, the valve is closed. The possibility of an unwanted interaction with residual gas molecules must also be investigated. The sticking coefficient of K atoms from the beam is unity, while the sticking on graphite for gas molecules is low. During the filling process, the high temperature and the short residence time for K on the surface Žsee above. means that the interaction between K atoms on the surface and any adsorbed molecules is negligible. When the graphite sample is at room temperature as during the experiments reported here, diffusion of K out of the bulk is likely to take place directly into the gas phase as K ) , over the barrier in the bulk measured in Ref. w10x. Thus, the surface interaction time is also very low in this case. How-
266
J. Wang, L. Holmlidr Chemical Physics Letters 325 (2000) 264–268
ever, energy transfer from the K ) atoms in the gas phase may give Rydberg states of residual gas molecules, in the same way as Rydberg states of other gas molecules are formed w12x. The inclusion of such molecules in the clusters can sometimes be observed as more complex and variable TOF patterns during the start of experimental runs.
3. Results and discussion Fig. 1 shows the TOF spectra for Kq N fragments following the nanosecond laser-induced photofragmentation at different sample voltages, i.e. at differ-
Fig. 1. Variation of ion signal with sample voltage. The arrows indicate the calculated positions of a mass peak with no kinetic energy release Žstart of arrow. and an energy release of 0.4 eV Žtip of arrow.. The full curves are with horizontal laser polarization, thus with kinetic energy release, and the broken curves are with vertical polarization. Some preformed cluster dips are indicated to the right. Observe that in the curves at 10 and 12.5 V sample q voltage the largest ion cluster is Kq 3 , not K 4 as otherwise.
ent field strengths and accelerating voltages for the ions. The broken curves are found with vertical polarization of the laser, which means that any Coulomb explosions will take place in the vertical directions, with no ions with kinetic energy excess in the direction towards the detector. Thus, the peaks found in these curves are due to cluster ions with close to the nominal acceleration voltage. The full curves in Fig. 1 show the results with horizontal laser polarization, which means that kinetic energy release in Coulomb explosions may give fast ions, as also seen in Fig. 1. The data in general agree with an excess energy of 0.4 eV. Since there are also ion formation processes which do not give an excess energy, like removal of one electron from a neutral cluster, cluster ions with zero excess energy are also observed with horizontal polarization. The arrows in the figure indicate the calculated positions of various ions without Žstart of arrow. and with 0.4 eV excess energy Žtip of arrow.. The ions which have a kinetic q q q energy release ŽKER. are clearly Kq 2, K3, K4, K7 q and probably also K 1 . In Fig. 1, some large neutral Rydberg Matter clusters are also identified and indicated. It is likely that the small ions are fragments from these large neutral clusters. In Fig. 1, it is very clear that the mass spectra Žcluster ion distributions. change strongly with the field strength in the range of 1–2 Vrcm. This indicates directly that Rydberg states are responsible for the spectra, since ordinary molecules or ground state clusters will not be sensitive to such field strengths. In Fig. 1 a change in fragmentation pattern is seen when the field strength of 1.5 Vrcm is reached, with q Kq 4 being the main fragment ion instead of K 3 . When the field strength reaches 2.2 Vrcm, the Kq 1 q and Kq 2 peaks disappear, and K 7 becomes the largest ion. This ion still appears to be a fragment ion, since there is a shift in energy between the two polarization directions. These changes in fragmentation pattern are probably due to the decreased stability of the small Rydberg state cluster ions in the strong fields. The result of laser fluence changes is shown in Fig. 2 at a field strength of 0.3 Vrcm, i.e. at a sample voltage of 2.5 V. The fluence increases upwards in Fig. 2, and it is seen that the fragment ion Kq 2 receives the same kinetic energy, more precisely determined in this set of data to be 0.35 eV, indepen-
J. Wang, L. Holmlidr Chemical Physics Letters 325 (2000) 264–268
Ž Fig. 2. Kq 2 signal with horizontal laser polarization upper curves in each pair. and vertical polarization Žlower curves.. The three pairs of curves are taken with increasing laser fluence from bottom up, and they are arbitrarily displaced for better visibility. The unmarked broad center peak is Kq 2 , and the front and back peaks correspond to initial kinetic energy release of 0.35 eV towards or away from the detector. The center peak also contains K) 2 . Sample voltage was 2.5 eV.
dent of the fluence. However, the intensity of the Kq 2 ion with excess kinetic energy directed towards the detector increases with increasing laser fluence. There are two further, quite important features in Fig. 2. One of them is that there should also exist fragment ions with initial velocity due to the KER in the direction pointing away from the detector. Such ions Kq 2 should appear at 95 m s. Unfortunately, this is also the time position of the Kq 4 ion with no KER, and the contributions cannot be conveniently separated. However, in the experiment in Fig. 1 it is clear that the Kq 4 peak is shifted to slightly shorter flight times when the laser has horizontal polarization. In Fig. 2, the contrary is true, which may indicate a considerable contribution from the Kq 2 ion initially moving backwards. Another aspect of the results in Fig. 2 is that the center peak, which agrees with Kq 2 with no excess energy, is larger with horizontal polarization. A further analysis shows that this broad peak also coincides with the neutral cluster K )2 with the same excess energy of 0.35 eV, which explains its broad peak shape. In Fig. 1, a broad distribution is also seen at the lowest field strengths, centered around 70
267
ms which is the calculated flight time of these Rydberg clusters. That these neutral clusters are in Rydberg states is obvious since they can be detected here. Typically, the lasers used to study Coulomb explosions in ground state molecules and clusters have extremely intense laser field with intensities of 10 13 – 10 15 W cmy2 in their 100 fs pulses w2–4x. The intense laser fields are then as strong as the internal fields that bind the outer electrons in the molecules. However, the ion fragments obtained in our measurements are produced by a laser with much lower intensity, less than 10 9 W cmy2 . The observed energy release in the present study is also low, 0.4 eV in contrast to the KER for molecules which are of the order of 6-8 eV. The fragment ions repel each other in the Coulomb explosion due to the force between the charged fragments. This results in photofragmentation and the ionic fragments have initial kinetic energies. One aspect of the results obtained with a nanosecond laser is that the clusters do not rotate appreciably during absorption of the photons required to strip off two electrons, probably at least two photons. If they rotated considerably during the pulse duration, a strong polarization effect would not be seen. Also, the cluster does not rotate during the fragmentation, but this is a fast process. However, a small ground state cluster would rotate with a period of 1 ps, very short relative to the 5 ns pulse length during which the photons are absorbed. This puts a lower limit on the rotation period, and thus on the size of the cluster. A simple estimate gives a minimum size of the cluster of 0.3 mm, but since the geometry of the parent clusters is not known, this is just an estimate. The excess kinetic energy release is the repulsion energy expected between two positive ions at a short distance which would be released in a Coulomb explosion in a cluster w6x. An excess energy of 0.35 eV results from an interionic distance of 4 nm. Since higher KER is not observed, it is reasonable to associate this distance with the shortest distance between the ions in the Rydberg Matter. This shortest distance is then the binding distance in the clusters. Using the classical theoretical results from Ref. w13x, such an interionic distance corresponds to a principal quantum number in the Rydberg Matter of n s 5.
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J. Wang, L. Holmlidr Chemical Physics Letters 325 (2000) 264–268
4. Conclusions
References
We have here reported studies of kinetic energy release of the order of 0.4 eV in Coulomb explosions of Rydberg Matter clusters K N . The fragment ions obtained are Kq N with N s 1, 2, 3, 4 and 7. The laser fragmentation is performed at power densities - 10 9 W cmy2 , and the fragmentation pattern varies with the external electric field strength at 1–3 Vrcm. This low laser intensity and the electric field sensitivity shows conclusively that the parent clusters are in Rydberg states or, as the TOF-MS data indicate, in Rydberg Matter form. The excess kinetic energy corresponds to an interionic distance in the Coulomb explosions of 4 nm. The existence of the polarization effect for ns pulses indicates no rotation during the excitation and fragmentation processes, and thus the parent clusters are large, at least of the order of 0.3 mm. The interionic distance and the cluster size found do not agree with ground state clusters but indicate instead Rydberg Matter clusters.
w1x J. Wang, L. Holmlid, Chem. Phys. Lett. 295 Ž1998. 500. w2x J.H. Posthumus, J. Plumridge, M.K. Thomas, K. Codling, L.J. Frasinski, A.J. Langley, P.F. Taday, J. Phys. B: At. Mol. Opt. Phys. 31 Ž1998. L553. w3x J.H. Posthumus, A.J. Giles, M.R. Thompson, K. Codling, J. Phys. B: At. Mol. Opt. Phys. 29 Ž1996. 5811. w4x J.H. Posthumus, A.J. Giles, M.R. Thompson, W. Shaikh, A.J. Langley, L.J. Frasinski, K. Codling, J. Phys. B: At. Mol. Opt. Phys. 29 Ž1996. L525. w5x S. Banerjee, G. Ravindra Kumar, D. Mathur, J. Phys. B: At. Mol. Opt. Phys. 32 Ž1999. L305. w6x O. Schapiro, P.J. Kuntz, K. Moehring, P.A. Hervieux, D.H.E. Gross, M.E. Madjet, Z. Phys. D 41 Ž1997. 219. w7x E.E.B. Campbell, in: Clusters of Atoms and Molecules, H. Haberland ŽEd.., Springer, Berlin, 1994. w8x J. Wang, R. Andersson, L. Holmlid, Surf. Sci. 399 Ž1998. L337. w9x M. Andersson, J. Wang, L. Holmlid, J. Chem. Soc. Faraday Trans. 92 Ž1996. 4581. w10x L. Holmlid, J. Phys. Chem. A 102 Ž1998. 10636. w11x L. Holmlid, J. Wang, Chem. Phys. Lett. 268 Ž1997. 285. w12x J. Wang, L. Holmlid, submitted. w13x L. Holmlid, Chem. Phys. 237 Ž1998. 11.
Chemical Physics Letters 325 Ž2000. 264–268 www.elsevier.nlrlocatercplett
Polarization effects in laser photofragmentation of Rydberg Matter clusters K )N in a weak electric field Jiaxi Wang 1, Leif Holmlid ) Reaction Dynamics Group, Department of Chemistry, Goteborg UniÕersity, SE-412 96 Goteborg, Sweden ¨ ¨ Received 4 January 2000; in final form 20 March 2000
Abstract The kinetic energy release and its variation with laser polarization in laser fragmentation of Rydberg Matter clusters is studied with a power density - 10 9 W cmy2 . The fragment ions observed are Kq N with N s 1, 2, 3, 4 and 7. The excess kinetic energy of the fragment ions observed with the laser polarization in the detector plane is approximately 0.4 eV. The fragmentation is sensitive to the external field strength at 1–3 V cmy1. The interionic distance in the Coulomb explosions is 4 nm. The polarization effect for ns pulses indicates slow cluster rotation and a cluster size of ) 0.3 mm. q 2000 Published by Elsevier Science B.V.
1. Introduction We have recently developed a laser-based method to investigate the formation of Rydberg and Rydberg Matter clusters at solid surfaces w1x. We now apply it to the study of polarization effects in the laser-induced fragmentation of small Rydberg Matter clusters, K N , formed outside a graphite basal surface. The ion time-of-flight ŽTOF. spectra vary clearly with the polarization direction of the laser, with a well separated fast ion peak when the laser polarization is in the same plane as the detector. Such effects are usually understood as a fragmentation induced by field ionization of the cluster, during which process
) Corresponding author: Fax: q46-31 7723107; e-mail [email protected] 1 Present address. Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ont. M5S 1A1, Canada.
two or more electrons are stripped off the cluster or molecule. Following the field ionization, so-called Coulomb explosions take place when the bonding in the cluster or molecule is replaced by the repulsion between two positive ion charges. The use of lasers with pulse lengths of 100 fs or less has in the last few years made it possible to fragment ground state molecules by such electron stripping giving Coulomb explosions. For example, the Coulomb explosions of small molecules like I 2 , N2 and H 2 were studied by Posthumus et al. w2–4x and by Banerjee et al. w5x. In the case of N2 and H 2 it was concluded that the molecules were forced into so-called dynamic alignment by the laser field. Several groups also study similar processes in clusters. Na clusters have been studied by Schapiro et al. w6x, and a microcanonical description was used to interpret the results in terms of phase transitions in the clusters. For a review of cluster ionization and fragmentation, see Ref. w7x.
0009-2614r00r$ - see front matter q 2000 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 6 4 0 - 0
J. Wang, L. Holmlidr Chemical Physics Letters 325 (2000) 264–268
2. Experimental The experiments are carried out in an UHV apparatus with a base pressure of 1 = 10y8 mbar. This has been fully described previously w8,9x. The pyrolytic graphite sample in the center of the UHV chamber was cut from a crystal ŽGrade ZYB, Advanced Ceramics Corp.. as a rod with a thickness of 0.8 mm, a width of 3.3 mm, and a length of 30 mm. It is mounted in the vertical direction, clamped at the ends by Ta foils, exposing a length of 16 mm of its basal Ž0001. surface. The sample is at room temperature in the laser experiments. The sample is filled with K from a molecular beam as described below, usually the day before the TOF experiments. As shown in Ref. w10x, the desorption and emission of K from a graphite surface gives K ) Rydberg states. These Rydberg states condense to form a cloud of Rydberg Matter outside the sample, from which well-defined clusters can be released by laser pulses w1x. A Nd:YAG laser with a maximum average power of 1 W at 532 nm wavelength and 7 ns pulses with a 10 Hz repetition rate is used to pump the dye laser ŽLambda-Physik.. Rhodamine 6G is used to give radiation at 564.0 nm in 5 ns pulses at a maximum average power at the UHV chamber of 100 mW. The laser beam diameter is approximately 3 mm without focusing, and the corresponding power density is less than 1.5 W cmy2 . A single lens with a focal length of 0.6 m is used to focus the laser beam. With focusing, the peak power density in the pulse is less than 10 9 W cmy2 , corresponding to a beam waist diameter of 0.4 mm. The laser beam enters through a window in the vacuum chamber and passes in front of the graphite sample at a distance of 5–15 mm. Its plane of polarization can be changed by rotating a half-wave plate in the laser beam, positioned close to the vacuum window to avoid any unwanted motion of the laser beam during the change of polarization direction. The final dichroic mirror after the halfwave plate does not change its reflectance appreciably for the two polarization directions. The detector box is placed 8.5 cm in front of the sample. The voltages on the sample are 2.5–20 V, corresponding to approximately 0.5–4 Vrcm at the laser beam position. The only particles which can be measured by the detector are positive ions, and Rydberg states
265
which field ionize in the detector to give positive ions. The ions are drawn down into a channel electron multiplier where they are amplified. The resulting pulses are further amplified, brought through a discriminator and collected in a multi-channel analyzer. Usually, 500 periods of the signal are averaged and output to a computer. To fill the graphite sample with K atoms, a K molecular beam is directed towards the graphite sample at an angle of 458 relative to the normal of the exposed basal surface, while the voltage of the sample is y10 V and its temperature is raised to 1500 K by passing an AC current of approximately 30 A through it. The K beam is a thermal beam from a two-chamber source, with the reservoir at approximately 525 K and the front of the source 50–100 K hotter. The source chamber is separately pumped, with one further intermediate differential stage to decrease the gas load to the UHV sample chamber. The impinging beam flux density is of the order of 5 = 10y8 A cmy2 or 3 = 10 11 atoms cmy2 sy1 at the sample, corresponding to 1 = 10y9 mbar pressure. At this flux density, a monolayer would be formed in 1 h if no desorption or diffusion into the sample took place. If only desorption took place, the surface density of K would be 10y1 0 of a monolayer, using rates of desorption from Ref. w10x. However, the diffusion into the bulk is much faster than the desorption w11x. The time spent for each K atom on the surface before diffusing into the bulk is estimated to be - 100 m s. A beam flag in front of the source and a valve between the source chamber and the sample chamber can be used to turn off the beam. During the laser experiments, the valve is closed. The possibility of an unwanted interaction with residual gas molecules must also be investigated. The sticking coefficient of K atoms from the beam is unity, while the sticking on graphite for gas molecules is low. During the filling process, the high temperature and the short residence time for K on the surface Žsee above. means that the interaction between K atoms on the surface and any adsorbed molecules is negligible. When the graphite sample is at room temperature as during the experiments reported here, diffusion of K out of the bulk is likely to take place directly into the gas phase as K ) , over the barrier in the bulk measured in Ref. w10x. Thus, the surface interaction time is also very low in this case. How-
266
J. Wang, L. Holmlidr Chemical Physics Letters 325 (2000) 264–268
ever, energy transfer from the K ) atoms in the gas phase may give Rydberg states of residual gas molecules, in the same way as Rydberg states of other gas molecules are formed w12x. The inclusion of such molecules in the clusters can sometimes be observed as more complex and variable TOF patterns during the start of experimental runs.
3. Results and discussion Fig. 1 shows the TOF spectra for Kq N fragments following the nanosecond laser-induced photofragmentation at different sample voltages, i.e. at differ-
Fig. 1. Variation of ion signal with sample voltage. The arrows indicate the calculated positions of a mass peak with no kinetic energy release Žstart of arrow. and an energy release of 0.4 eV Žtip of arrow.. The full curves are with horizontal laser polarization, thus with kinetic energy release, and the broken curves are with vertical polarization. Some preformed cluster dips are indicated to the right. Observe that in the curves at 10 and 12.5 V sample q voltage the largest ion cluster is Kq 3 , not K 4 as otherwise.
ent field strengths and accelerating voltages for the ions. The broken curves are found with vertical polarization of the laser, which means that any Coulomb explosions will take place in the vertical directions, with no ions with kinetic energy excess in the direction towards the detector. Thus, the peaks found in these curves are due to cluster ions with close to the nominal acceleration voltage. The full curves in Fig. 1 show the results with horizontal laser polarization, which means that kinetic energy release in Coulomb explosions may give fast ions, as also seen in Fig. 1. The data in general agree with an excess energy of 0.4 eV. Since there are also ion formation processes which do not give an excess energy, like removal of one electron from a neutral cluster, cluster ions with zero excess energy are also observed with horizontal polarization. The arrows in the figure indicate the calculated positions of various ions without Žstart of arrow. and with 0.4 eV excess energy Žtip of arrow.. The ions which have a kinetic q q q energy release ŽKER. are clearly Kq 2, K3, K4, K7 q and probably also K 1 . In Fig. 1, some large neutral Rydberg Matter clusters are also identified and indicated. It is likely that the small ions are fragments from these large neutral clusters. In Fig. 1, it is very clear that the mass spectra Žcluster ion distributions. change strongly with the field strength in the range of 1–2 Vrcm. This indicates directly that Rydberg states are responsible for the spectra, since ordinary molecules or ground state clusters will not be sensitive to such field strengths. In Fig. 1 a change in fragmentation pattern is seen when the field strength of 1.5 Vrcm is reached, with q Kq 4 being the main fragment ion instead of K 3 . When the field strength reaches 2.2 Vrcm, the Kq 1 q and Kq 2 peaks disappear, and K 7 becomes the largest ion. This ion still appears to be a fragment ion, since there is a shift in energy between the two polarization directions. These changes in fragmentation pattern are probably due to the decreased stability of the small Rydberg state cluster ions in the strong fields. The result of laser fluence changes is shown in Fig. 2 at a field strength of 0.3 Vrcm, i.e. at a sample voltage of 2.5 V. The fluence increases upwards in Fig. 2, and it is seen that the fragment ion Kq 2 receives the same kinetic energy, more precisely determined in this set of data to be 0.35 eV, indepen-
J. Wang, L. Holmlidr Chemical Physics Letters 325 (2000) 264–268
Ž Fig. 2. Kq 2 signal with horizontal laser polarization upper curves in each pair. and vertical polarization Žlower curves.. The three pairs of curves are taken with increasing laser fluence from bottom up, and they are arbitrarily displaced for better visibility. The unmarked broad center peak is Kq 2 , and the front and back peaks correspond to initial kinetic energy release of 0.35 eV towards or away from the detector. The center peak also contains K) 2 . Sample voltage was 2.5 eV.
dent of the fluence. However, the intensity of the Kq 2 ion with excess kinetic energy directed towards the detector increases with increasing laser fluence. There are two further, quite important features in Fig. 2. One of them is that there should also exist fragment ions with initial velocity due to the KER in the direction pointing away from the detector. Such ions Kq 2 should appear at 95 m s. Unfortunately, this is also the time position of the Kq 4 ion with no KER, and the contributions cannot be conveniently separated. However, in the experiment in Fig. 1 it is clear that the Kq 4 peak is shifted to slightly shorter flight times when the laser has horizontal polarization. In Fig. 2, the contrary is true, which may indicate a considerable contribution from the Kq 2 ion initially moving backwards. Another aspect of the results in Fig. 2 is that the center peak, which agrees with Kq 2 with no excess energy, is larger with horizontal polarization. A further analysis shows that this broad peak also coincides with the neutral cluster K )2 with the same excess energy of 0.35 eV, which explains its broad peak shape. In Fig. 1, a broad distribution is also seen at the lowest field strengths, centered around 70
267
ms which is the calculated flight time of these Rydberg clusters. That these neutral clusters are in Rydberg states is obvious since they can be detected here. Typically, the lasers used to study Coulomb explosions in ground state molecules and clusters have extremely intense laser field with intensities of 10 13 – 10 15 W cmy2 in their 100 fs pulses w2–4x. The intense laser fields are then as strong as the internal fields that bind the outer electrons in the molecules. However, the ion fragments obtained in our measurements are produced by a laser with much lower intensity, less than 10 9 W cmy2 . The observed energy release in the present study is also low, 0.4 eV in contrast to the KER for molecules which are of the order of 6-8 eV. The fragment ions repel each other in the Coulomb explosion due to the force between the charged fragments. This results in photofragmentation and the ionic fragments have initial kinetic energies. One aspect of the results obtained with a nanosecond laser is that the clusters do not rotate appreciably during absorption of the photons required to strip off two electrons, probably at least two photons. If they rotated considerably during the pulse duration, a strong polarization effect would not be seen. Also, the cluster does not rotate during the fragmentation, but this is a fast process. However, a small ground state cluster would rotate with a period of 1 ps, very short relative to the 5 ns pulse length during which the photons are absorbed. This puts a lower limit on the rotation period, and thus on the size of the cluster. A simple estimate gives a minimum size of the cluster of 0.3 mm, but since the geometry of the parent clusters is not known, this is just an estimate. The excess kinetic energy release is the repulsion energy expected between two positive ions at a short distance which would be released in a Coulomb explosion in a cluster w6x. An excess energy of 0.35 eV results from an interionic distance of 4 nm. Since higher KER is not observed, it is reasonable to associate this distance with the shortest distance between the ions in the Rydberg Matter. This shortest distance is then the binding distance in the clusters. Using the classical theoretical results from Ref. w13x, such an interionic distance corresponds to a principal quantum number in the Rydberg Matter of n s 5.
268
J. Wang, L. Holmlidr Chemical Physics Letters 325 (2000) 264–268
4. Conclusions
References
We have here reported studies of kinetic energy release of the order of 0.4 eV in Coulomb explosions of Rydberg Matter clusters K N . The fragment ions obtained are Kq N with N s 1, 2, 3, 4 and 7. The laser fragmentation is performed at power densities - 10 9 W cmy2 , and the fragmentation pattern varies with the external electric field strength at 1–3 Vrcm. This low laser intensity and the electric field sensitivity shows conclusively that the parent clusters are in Rydberg states or, as the TOF-MS data indicate, in Rydberg Matter form. The excess kinetic energy corresponds to an interionic distance in the Coulomb explosions of 4 nm. The existence of the polarization effect for ns pulses indicates no rotation during the excitation and fragmentation processes, and thus the parent clusters are large, at least of the order of 0.3 mm. The interionic distance and the cluster size found do not agree with ground state clusters but indicate instead Rydberg Matter clusters.
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