Recent Research
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§ Isomeric excitation of 229Th in laser-heated clusters
Efficient excitation of 229Th from the nuclear ground state to the 8-eV isomeric state remains a challenging problem to be solved. In our recent publication [1], we consider isomeric excitation of 229Th in a laser-heated cluster. A 229Th cluster is first radiated by an intense femtosecond laser pulse, causing ionization of the constituting atoms. The cluster will then survive for a time on the order of 1 ps, during which the electrons collide with the nuclei repeatedly and excite them to the isomeric state. Two mechanisms are responsible for the isomeric excitation: nuclear excitation by electron capture (NEEC) and nuclear excitation by inelastic electron scattering (NEIES). Other mechanisms, including nuclear excitation by bound-bound electronic transitions and photoexcitations, are estimated to be at least several orders of magnitude weaker and can be neglected. Our calculation shows that for relatively weak laser intensities around 1014 W/cm2, the excitation mechanism is dominantly NEEC. And for relatively strong laser intensities around 1016 W/cm2, the excitation mechanism is dominantly NEIES. One can tune between NEEC and NEIES continuously by changing the laser intensity. This laser-heated-cluster scheme not only provides an efficient means for isomeric excitation, but also provides an approach for the confirmation of the NEEC process. The NEEC process has been predicted theoretically for nearly half a century [2], but it has not been confirmed conclusively in experiments. Recently, two beam-based experiments aim to confirm NEEC with 93mMo, but contradicting results are reported [3, 4]. Laser-cluster interaction provides a laser-based, in contrast to the more traditional beam-based, approach for the experimental verification of the NEEC process.
References: [1] J. Qi, H. Zhang, and X. Wang, Phys. Rev. Lett. 130, 112501 (2023). [2] V. I. Goldanskii and V. A. Namiot, Phys. Lett. B 62, 393 (1976). [3] C. J. Chiara et al., Nature 554, 216 (2018). [4] S. Guo et al., Phys. Rev. Lett. 128, 242502 (2022).
§ 229Th nuclear physics and strong-field atomic physics
Among all known nuclei the 229Th nucleus has a unique low-lying isomeric state with an energy of 8 eV above the nuclear ground state. The existence of this isomeric state has fascinated the scientific community with potential applications of a "nuclear clock" or a "nuclear battery". How to excite the 229Th nucleus from the ground state to the isomeric state in a controllable and efficient way remains an open question and is also the first step towards the fascinating applications. We propose an excitation approach based on laser-driven electron recollision, as illustrated in the above figure. Recollision is the core process of strong-field atomic physics. It has been well understood that the recolliding electron may (i) recombine radiatively to the ion core leading to high harmonic generation; or (ii) kick out another electron leading to nonsequential double ionization; or (iii) elastically scattered by the ion core leading to laser-induced electron diffraction. With the 229Th nucleus, the recolliding electron has a new channel of exciting the nucleus. It is interesting to see the connection between strong-field atomic physics and nuclear physics with the 229Th system. Details can be found in our recent publication [1,2].
References: [1] W. Wang, J. Zhou, B. Liu, and X. Wang, Phys. Rev. Lett. 127, 052501 (2021). [2] X. Wang, Phys. Rev. C 106, 024606 (2022).
§ Laser-assisted deuteron-triton fusion
Controlled nuclear fusion has the potential of supplying sustainable and clean energy solutions. In either magnetic confinement fusion or inertial confinement fusion, which are two major schemes for controlled fusion research, the deuteron-triton (DT) fusion reaction is chosen for its relatively high reaction cross sections compared to other fusion options. Even so, the required temperature is very high, usually on the order of 10–100 million K in order to attain appreciable fusion reaction probabilities. These temperatures are very challenging to achieve and maintain. New methods or tools that can further increase the DT reaction probabilities and relax the demanding temperature requirement are, therefore, particularly desirable. In our recent publication [1], we show that intense laser fields, especially those with relatively low frequencie, are highly effective in transferring energy to the DT system and enhancing the fusion probability. The enhancement can be orders of magnitude, as shown in the above figures. E is the energy of relative motion between D and T without external laser fields. The laser-assisted fusion cross section is shown with three wavelengths (800 nm, 400 nm, and 100 nm) at different intensities in comparison to the corresponding laser-free cross section. See [1] for detailed analyses.
Reference: [1] X. Wang, Phys. Rev. C 102, 011601(R) (2020).
§ Recollision with circularly polarized laser fields: The first experimental confirmation It has been predicted by quite a few theoretical works that recollision and nonsequential double ionization (NSDI) are possible with elliptically polarized (EP) or circularly polarized (CP) laser fields [1-4]. The only experimental result that seems to support these predictions is a "knee structure" observed in the double ionization of magnesium (Mg) with CP laser fields [5]. With the single data of [5], however, it is not possible to confirm that the observed knee structure is actually the result of recollision, especially considering that the experimental condition was in the multiphoton regime. It is possible that the observed knee structure of Mg is the result of other mechanisms. In a recently published paper [6], due to a collaborative effort from the experimental group of Prof. L. F. DiMauro at the Ohio State University and our group, we experimentally confirm that the NSDI observed in Mg with CP laser fields is the result of recollision. The confirmation is achieved in the following sense: If the recollision interpretation were correct, then the observed NSDI yield would change with the laser wavelength, the laser ellipticity, and the atomic ionization potential in some predicted way. And the experimental results agree with all the predictions.
The recollision interpretation predicts that the NSDI yield decreases with the increase of laser wavelength. This is confirmed experimentally, as shown in Fig. 1 above. By increasing the laser wavelength from 800 nm (a) to 1030 nm (c), the double ionization knee structure disappears. A similar trend is shown in classical simulation results, as shown in (b,d). The recollision interpretation predicts that the NSDI yield prefers lower laser ellipticity values. This is confirmed by the experiment as shown in Fig. 2(a) below. Changing the laser ellipticity from 1.0 to 0.75, the NSDI yield increases. Classical simulation results are shown in 2(b) for comparison. The recollision interpretation also predicts that the NSDI yield prefers atoms with lower ionization potentials. This is confirmed by the experiment as shown in Fig. 3(a) below. Changing the atomic target from Mg (Ip=7.64 eV) to Zn (Ip=9.39 eV), the NSDI yields decreases. Classical simulation results are shown in 3(b) for comparison.
In conclusion, we provide the first experimental confirmation of the ocurrence of recollision with circular polarization. More details can be found in our paper [6].
References: [1] X. Wang and J. H. Eberly, Phys. Rev. Lett. 105, 083001 (2010). [2] X. Wang and J. H. Eberly, New J. Phys. 12, 093047 (2010). [3] F. Mauger, C. Chandre, and T. Uzer, Phys. Rev. Lett. 105, 083002 (2010). [4] L. B. Fu, G. G. Xin, D. F. Ye, and J. Liu, Phys. Rev. Lett. 108, 103601 (2012). [5] G. D. Gillen, M. A. Walker, and L. D. Van Woerkom, Phys. Rev. A 64, 043413 (2001). [6] Y. H. Lai, X. Wang*, Y. Li, X. Gong, B. K. Talbert, C. I. Blaga, P. Agostini, and L. F. DiMauro*, Phys. Rev. A 101, 013405 (2020).
§ Alpha decay in intense laser fields
Lasers are getting more and more intense. Intensities on the order of 1021 W/cm2 can be routinely generated in laser-fusion facilities. Intensities on the order of 1024 W/cm2 are expected in the forthcoming years with the ELI facility of Europe. One may estimate that the laser electric field corresponding to such an intensity is comparable to the Coulomb electric field not far (~10 fm) from the nucleus. Therefore the laser field may directly influence nuclear properties or processes, opening the door to "strong-field nuclear physics". In our recent article [1], we consider how much the alpha decay process may be affected by an intense laser field. The alpha decay process has been understood as a quantum tunneling process, which depends very sensitively on the shape of the alpha-nucleus potential barrier. The intense laser field modifies the alpha decay process via modifying the potential barrier. Our calculation shows that with an intensity of 1024 W/cm2, alpha decay can be modified by a small yet finite amount, on the order of 0.1%. We also predict that alpha decays with lower decay energies are relatively easier to be modified than those with higher decay energies, due to longer tunneling paths for the laser field to act on.
Reference: [1] J. Qi, T. Li, R. Xu, L. Fu, and X. Wang, Phys. Rev. C 99, 044610 (2019).
§ Tunneling exit momentum: Zero? Nonzero?
Tunneling is a quantum mechanical effect that exists in various physical systems. Strong-field atomic ionization is one of them. In a strong laser field, the potential energy felt by the electron is a tilted Coulomb potential, forming a potential barrier through which the electron can tunnel out from inside the atom. We know that initially the electron is inside the atom. We know that finally it is outside the atom. However, what happens in between, i.e., how exactly the tunneling process happens, is very controversial. One of the controvesial questions is: what is the momentum (velocity) of the electron at the tunneling exit, where it has just finished the tunneling process? Many think that it is zero, because at the tunneling exit the potential energy equals to the total energy hence the kinentic energy of the particle is zero. This conjecture has actually been used in many places. Interestingly, whether this conjecture is true can be checked by strong-field experiments. The tunneling exit momentum can leave its footprint in the measurable end-of-pulse momentum. In 2012, the experiment by ETH Zurich [1] says that this tunneling exit momentum has a quite large deviation (~0.8 a.u.) from zero. But a later experiment in 2014 by Peking University [2] disagrees and says that the tunneling exit momentum is very close to zero (< 0.1 a.u.). These two experiments have been puzzling people for a few years. This puzzle has been solved by us in a recent paper published in PRL [3]. By calculating the flow velocity of the probability fluid at the tunneling exit, we show that the experiments by ETH Zurich and by Peking University do not contradict each other. They reach different conclusions because they use different atoms and laser parameters. That is to say, the tunneling exit momentum is not some constant quantity (like zero), instead, it depends on the atomic target as well as the laser parameters. It could be close to zero under some condition, or could be very different from zero under another. The kinetic energy is zero at the tunneling exit for sure. In quantum mechanics, the kinetic energy operator is proportional to the 2nd-order derivative in space while the momentum operator is to the 1st-order derivative. The 2nd-order derivative being zero does not tell anything about the value of the 1st-order derivative.
References: [1] A. N. Pfeiffer, C. Cirelli, A. S. Landsman, M. Smolarski,
D. Dimitrovski, L. B. Madsen, and U. Keller, Phys. Rev.
Lett. 109, 083002 (2012)
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