Laser-Field Induced Modifications of Electron-Transfer Processes in

Apr 20, 2017 - A single-color laser-induced collisional charge transfer system of Xe+–N2 is introduced, and the charge transfer process between Xe+ ...
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Laser-Field Induced Modifications of Electron-Transfer Processes in Xe+−N2 Collision Zhenzhong Lu,* Yanling Sun, Lin Ma, and Jifang Liu School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071,China S Supporting Information *

ABSTRACT: A single-color laser-induced collisional charge transfer system of Xe+−N2 is introduced, and the charge transfer process between Xe+ and N2 is observed. The laser-induced charge transfer (LICT) in Xe+−N2 system is experimentally investigated through time-of-flight (TOF) mass spectrometry, and the intensity dependence of product ions at ∼440 nm under several different conditions are measured, showing the impacts of laser parameters, mixing ratio, and pressure on the charge transfer process. Our results indicate that the charge transfer process between Xe+ and N2 can be enhanced by the joint action of the collisional and radiative interactions.

1. INTRODUCTION Charge transfer is an important mechanism in establishing the ionization structure of a plasma containing multiply charged ionic systems.1−3 The relevant collision-induced reactions in the absence of a laser field such as the charge-transfer reactions of N2+(v) + Ar → N2 + Ar+,4−8 Ar+ + NO → Ar + NO+, Ar+ + O2 → Ar + O2+, and Ar+ + CO → Ar + CO+,9 have been extensively studied because they play a major role in plasmas, as well as in astrophysics. However, the charge-transfer process is severely limited by the energy defect between the collisional particles, especially in the low-collisional-energy regime. In general it is expected that only exothermic transitions, i.e., the energy of the reactants is larger than the energy of the products, will have a considerable reaction probability. However, the laser-induced collision process can overcome the energy defect factor, and the cross section of the collision process will be increased greatly. In the laser-induced charge transfer process, energy is first stored in the form of ions of one species (energy storage particle). An intense laser field is then used to transfer this energy rapidly and selectively to an ionic state of a second species (target particle). In the absence of either the collision or the laser field, the reaction is rigorously forbidden. The process of laser-induced charge transfer (LICT) has been investigated since the 1970s.10,11 The LICT is an optical process in which a laser field is used during the collision between two different particles to induce selective energy transfer from a level in one species to another level in a different species. On one hand, energy transfer between different particles is made more quickly and efficiently because of the participation of the laser field; on the other hand, the effect of interparticle collisions can implement transfers that are difficult for single photon excitation, thus can obtain the expected high excited state and consequently short wavelength laser. Therefore, to study laser-induced collision process is of great significance for development of short wavelength laser sources. In addition, the ability to excite the special target level © XXXX American Chemical Society

of the chosen particle permits potential applications in controlling pathways of chemical reactions. LICT calculations have been performed on simple systems: a completely stripped ion and a hydrogen atom (C6+ + H+ + ℏω12), He2+ + H + ℏω,13 or two alkali elements (K+ + Na + ℏω14) in the case of the weak-field limit, for which laser coupling can be regarded as a perturbation. In the slow-collision regime, the process is described in the molecular-state scheme and the reliability of different approximations is checked including that of the useful Landau−Zener model. Recently, the progresses in ultrafast intense laser techniques have made it possible to probe collision and laser-induced process in new regimes. In more recent works, people have focused on laser intensities above 1013 W/cm2, and many theoretical models have been proposed. Unfortunately these reactions are not very well suited to experiments. So far, only a few experiments of laser-induced collision between ion and atom have been performed such as Ca+ + Sr + ℏω → Ca + Sr+15 and Ne+ + He + ℏω → Ne + He+ collision.16 In this work, a novel single-color laser-induced ion−molecule collisional charge transfer system is proposed and laser-induced charge transfer in Xe+ + N2 collisions is observed. Considering the reaction Xe+ + N2 → Xe + N2+ from the point of the gases ionizing potential since the ionization potentials of the Xe and N2 are 13.4 and 15.6 eV, respectively, the direct transition Xe+ + N2 → Xe + N2+ is energetically forbidden; consequently, the transition can take place only in the presence of the radiation field. In our experiment, Xe atom is ionized by the multiphoton resonant ionization process to form Xe+ prior to the collision interaction. During the collision process between Xe+ and N2, because of the simultaneous effects of both collision and the laser field, the energy of Xe+ is transferred to N2, which at the Received: February 9, 2017 Revised: April 18, 2017 Published: April 20, 2017 A

DOI: 10.1021/acs.jpcc.7b01288 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

3. RESULTS AND DISCUSSION To prepare the initial state of the charge transfer system, multiphoton resonant ionization of atomic Xe is experimentally accomplished using ∼440 nm dye laser. Commonly used ways to ionize atomic xenon include ionization in intense field by short pulse laser18,19 and multiphoton ionization by laser of a wide wavelength range.20,21 However, for the simple generation of single ionic xenon, ion signals created by the former method are too abundant for the product ions to be controlled, so that the latter one is chosen to prepare Xe+ in our experiment. Because of the special level structure of atomic Xe that there are two states, the 4f state and the 6s state, that are resonant with several ∼440 nm laser photons, therefore resonance enhanced multiphoton ionization (REMPI) of atomic Xe by ∼440 nm laser has attracted a lot of interest22−24 and is also used in our experiment. The multiphoton resonant ionization of xenon at ∼440 nm has been discussed previously.25 For the Xe+−N2 laser-induced collisional charge transfer system, absorption of an additional photon at the same wavelength (i.e., the pump laser and the transfer laser are of the same frequency) will make N2 ionized; therefore, the laser-induced collision process can be accomplished with only one laser beam. Figure 2a shows the typical ion signals generated through multiphoton ionization of atomic xenon with a ∼440 nm dye

same time is ionized because of loss of an outer-shell electron, while Xe+ transits to its atomic ground state. The process also can be described by the following expression: Xe+ + N2 + ℏω → Xe + N2+

(1)

2. EXPERIMENTAL SETUP The experimental setup for detecting N2+ created by LICT process through time-of-flight (TOF) mass spectrometry is shown in Figure 1. A standard Wiley−McLaren TOF mass

Figure 1. Experimental setup of laser-induced charge transfer for the Xe+−N2 system.

spectrometer is mounted in the vacuum chamber with a residual background pressure of 5 × 10−5 Pa. A pulse valve jets mixed gases of Xe and N2. The gases are thoroughly mixed with specific volume ratios in the reservoir and also allowed to stand for some time after mixing. During the work process, the chamber pressure is maintained about 10−4−10−3 Pa. A dye laser (ScanmatePro, LAMBDA PHYSIK), pumped by the thirdharmonic generation (355 nm) of the Nd:YAG laser (Precision II 9010, Continuum), is used to generate laser output (10 Hz, 7 ns) tunable at the wavelength of ∼440 nm with a line width of 0.12 cm−1. The wavelength of the dye laser is tuned to nearresonate with the energy difference between the initial state Xe+ and final state N2+. The dye laser is then focused into the chamber by a quartz lens of 65 mm focal length to induce energy transfer. The pump and the transfer laser are of the same wavelength for this Xe+ + N2 system; only one laser beam is needed to accomplish the laser-induced collision process, which ordinarily requires two laser frequencies. A 4-channel Digital Delay/Pulse Generator DG535 is used to control the time sequence of the dye laser and the valve driver, keeping synchronization between the laser pulse and the mixture beam. The ions created from ionization region gain sufficient flight speed through the two accelerating electric fields of 800 and 1200 V, and then fly through a free flight region of 120.5 mm length to the Microchannel Plate (MCP) detector with a gain of 106. In the interaction region the impact energy is no more than the magnitude of 10 eV so that the ionization during the field-free collision is negligible.17 The ionization signals are observed directly by a 1 GHz DPO7104 digital oscilloscope (Tektronix), with the data stored on a computer disk for analysis.

Figure 2. Ion signals created by targeting pure Xe, pure N2, and Xe + N2 mixture, respectively.

laser. The product ions were detected with TOF mass spectrometry. For the pure N2, as shown in Figure 2b, N2 cannot be resonantly ionized directly with a ∼440 nm dye laser due to the strict selectivity of REMPI for the excitation wavelength. There is not an energy level that is (near) resonance with several ∼440 nm photons. In addition, the laser intensity used in our experiment was controlled to avoid the direct multiphoton ionization of N2. In contrast, when Xe atoms were mixed with the N2 in the reservoir, the signals of N2+ and N+ appear in the TOF mass spectrum (Figure 2c). The pressure was maintained at 2 × 10−4 Pa (the volume ratio of Xe/N2 was 0.3:1.1), while the pump laser wavelength was scanned from 438 to 441.5 nm. The spectra of Xe+, N2+, and N+ obtained from a Xe−N2 mixture in the vicinity of 440 nm was recorded with a Boxcar, and the spectral intensities are shown in Figure 3. The pulse energy of the dye laser used in the experiment was about 2.5 mJ. Ion B

DOI: 10.1021/acs.jpcc.7b01288 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

11:1, the peak intensities were measured again. The percentage of N+ was appreciably greater (Figure 4b), and the peak for N2+ was not observed. When the ratio of Xe/N2 increases to the point where Xe atoms far outnumber the N2 molecules, all of the N2+ will dissociate by collision of N2+ with Xe, and only N+ remains. To illustrate the influence of the volume ratio on the relative intensity of N+ and N2+ in detail, we studied the composition effect more closely (Figure 5). It is evident that

Figure 3. Intensity spectra of Xe+, N2+, and N+ in the vicinity of 440 nm.

spectra of N2+ and N+ have the same profile as the Xe+. The fact that no ion signal was observed beyond the studied wavelength range indicates that, when there is no Xe+, signals derived from N2+ and N + disappear. Under the same experimental conditions, with the pump laser wavelength scanned through the same range, N2+ and N+ signals could not be observed for pure N2 as shown in Figure 2b. It is apparent that the presence of Xe+ could directly enhance the ionization of N2, i.e., the formation of N2+ is the primary process that results from the charge transfer between Xe+ and N2. N+ is then formed by collision of unstable N2+ with other particles.26 Togness presented a detailed discussion on the ionization of nitrogen,26 and the results indicated that N2+ will dissociate to form N+ by collision of N2+ with gas molecules. In the present work, if the N+ is formed through the collisional dissociation of N2+, we can expect that when the collision probability of N2+ increases, the relative number of N+ will increase. The conclusion that N+ is formed by collision is further confirmed by the relative ion intensities in mixtures of Xe and N2. The peak intensities for N2+ and N+ were first obtained when the total pressure of the mixture was 1.05 × 10−3 Pa, and the volume ratio of Xe/N2 was 1.1:1 (Figure 4a). After the formation of N2+ by the LICT process, some of the N2+ can collide with atomic Xe to form N+. Both N2+ and N+ signals are observed in the TOF mass spectrum. While keeping the total pressure constant and increasing the volume ratio of Xe/N2 to

Figure 5. Percentage of N+ and N2+ as a function of volume ratio.

with the increased ratio, the percentage of N2+ tends toward zero. With the higher ratio, the resonant ionization of Xe reaches saturation; this means there are more residual Xe atoms, which increases the probability of collision of the unstable N2+ with Xe and its consequent disruption:23 N2+ = N+ + N. In Figure 5, it is evident that with the increased ratio, the percentage of N+ tends to be increasing, which is consistent with our prior expectation. Next consider the reaction Xe+ + N2 → Xe + N2+ from the gas’s ionizing potential point of view. Since the ionization potentials of Xe and N2 are 13.4 and 15.6 eV, respectively, the charge exchange reaction of Xe+ with N2 is not energetically allowed. It will thus be seen that in the absence of the laser field and at a very low impact velocity, the interchange is most probable in the case of Xe + N2+ → Xe+ + N2, which is the reverse reaction of Xe+ + N2 → Xe + N2+. This argument as given is not strictly applicable to the present conditions because

Figure 4. Ion signals created with different volume ratio. C

DOI: 10.1021/acs.jpcc.7b01288 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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laser field has a significant effect on the yield of N2+, and the charge transfer process between Xe+ and N2 is a result of a combined collision and radiative interaction acting together.

the present reaction combines collision with an external laser field, which can overcome the energy defect factor and realize the rapid and efficient charge transfer between Xe+ and N2. Therefore, the reaction Xe+ + N2 → Xe + N2+ needs the participation of the laser field to enhance the effect. Next to consider is the light interaction during the collisions. The laser-induced charge transfer cross section can be obtained by σexpt = (NN+ + N2+/NXe+)(1/NN2v ̅ τ )

4. CONCLUSION In conclusion, a single-color Xe+−N2 laser-induced collisional charge transfer system is introduced, and we have conducted a set of experiments to demonstrate the presence of laser-induced charge transfer in Xe+−N2 collision. Intensity of productions of N2+ and N+ were measured under several different conditions. Our results show that the presence of Xe+ could directly enhance the ionization of N2. In our experiment, the relative number of N+ increases with the increasing collision probability of N2+. When Xe atoms far outnumber the N2 molecules, all of the N2+ dissociate by collision of N2+ with Xe, and only N+ remains, which confirms the formation of N+ is formed by the dissociation of N2+. Moreover, the conversion ratio IN++N2+/IXe+ increases linearly with increased laser intensity, i.e., the cross section rises as the laser power density increases linearly. This relationship indicates that the laser field has a significant enhancement effect on the yield of N2+. Our experiment demonstrates that although the ionization potential of Xe (13.4 eV) is less than N2 (15.6 eV), such a reaction can be motivated by the use of a laser to modify the energy flow pathway. The LICT process can be an effective way to transfer energy selectively from a storage ionic state to a target ionic state. We hope that the LICT process discussed in this article will apply to other high-valence ions collisional systems to obtain the expected high excited state, which is important for the research of X-ray lasers. Besides, the ability to excite the special target level of the chosen particle by adjusting laser parameters such as wavelength, intensity, etc., permits applications of LICT in controlling pathways of chemical reactions, thus making a detailed understanding of atomic dynamical processes possible.

(2)

where v ̅ is the relative velocity and τ is the pulse width. The ground-state density of NN2 depends on the pressure of the reaction region and is independent of laser parameters. Thus, when we discuss the effect of laser field on the charge transfer process, NN2 can be taken as a constant. Although the densities in the interaction region cannot be precisely known, the observed IN++N2+/IXe+ is directly proportional to the ratio NN++N2+/NXe+. Therefore, the cross section σexpt is proportional to the ratio IN++N2+/IXe+, which reflects the ratio of Xe+ to N2+. The ratio IN++N2+/IXe+ as a function of laser intensity in the interaction region is shown in Figure 6 for the given conditions:

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01288. Time of flight and laser pulse energy (PDF)

Figure 6. Ionic yield as a function of laser intensity in the interaction region.

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laser wavelength of 440.5 nm, mixture pressure of 7.2 × 10−4 Pa, and a Xe/N2 ratio of 1:6. At such a low partial pressure, the ionization of Xe has reached saturation and the signal intensity of Xe+ obtained from pure Xe is almost constant at increasing laser intensities (see diamond points in Figure 6). If the ionization of N2 is due to collision between Xe+ and N2 without absorption of a transfer laser photon, IN++N2+/IXe+ should be independent of laser intensity. We should also note that LICT includes laser participation. The laser-induced charge transfer process in Xe+ and N2 is explained by the following mechanism. During the collision of Xe+ and N2, Xe+ energy is transferred to N2, which at the same time absorbs a transfer laser photon and is subsequently ionized. Absorption of a transfer laser photon is a switch that governs the energy flow from Xe+ to N2. It is a single-photon absorption process; thus, at the unsaturated region the cross section will rise as the laser power density increases linearly.12−15 IN++N2+/IXe+ as a function of laser intensity in the interaction region is shown in Figure 6. The conversion ratio IN++N2+/IXe+ increases linearly with increased laser intensity, i.e., the cross section rises as the laser power density increases linearly. This relationship indicates that the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhenzhong Lu: 0000-0002-5007-963X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 11304240 and 61378079) and the 111 Project (Grant No. B17035).

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