Xiao-Min Tong

Weak transitions between quantum states are of fundamental importance for a broad range of phenomena from analytical biochemistry to precision physics, but generally challenge experimental detection. Due to their small cross sections scaling with the absolute square of their transition matrix elements, spectroscopic measurements often fail in particular in the presence of competing background processes. Here we introduce a general concept to break this scaling law and enhance the transition probability by exploiting a stronger laser-coupled pathway to the same excited state. We demonstrate the concept experimentally by attosecond transient absorption spectroscopy in helium atoms. The quasi-forbidden transitions from the ground state 1s ² to the weakly coupled doubly excited 2p3d and s p _2,4− states are boosted by an order of magnitude. Enhancing single-photon-suppressed transitions can find widespread applicability, from spectral diagnostics of complex molecules in life and chemical sciences to precision spectroscopy of weak transitions in metastable atomic nuclei in the search for new physics.

A highly charged muonic ion is a unique few-body atomic system where a negatively charged muon and a few electrons are simultaneously bound to a single nucleus. We report the first state-selective observation of highly charged muonic Ar (μAr) by electronic K x-ray spectroscopy using an array of transition-edge sensor microcalorimeters. The high-precision K x-ray spectra provide a clear signature of the presence of muonic atoms with one, two, and three electrons, i.e., H-like, He-like, and Li-like μAr. With the aid of theoretical calculations, we confirmed that the peak positions are consistent with the x-ray energies from highly charged Cl ions, and the intensities reflect deexcitation dynamics of highly charged μAr.

We study the energy structures of muonic Ar atoms and find the muon orbital collapses at a critical angular momentum l_{c} using density-functional theory (DFT). The l_{c} may provide an upper limit for the muon-captured states in muon-Ar collisions. We confirm the existence of this upper limit by calculating the state-specified capture probability using the time-dependent Schrödinger equation (TDSE) and the classical trajectory Monte Carlo (CTMC) method with the single-active-particle approximation. Modifying the mapping between the classical orbital energy and the principal quantum number led to a reasonable agreement in the state-specified muon capture probabilities obtained by the TDSE and CTMC methods. We propose a simple method to estimate l_{c} for exotic noble atoms from atomic model potentials. The estimated values agree with those calculated by DFT.

Weak transitions between quantum states are of fundamental importance for a broad range of phenomena from analytical biochemistry to precision physics, but generally challenge experimental detection. Due to their small cross sections scaling with the absolute square of their transition matrix elements, spectroscopic measurements often fail in particular in the presence of competing background processes. Here we introduce a general concept to break this scaling law and enhance the transition probability by exploiting a stronger lasercoupled pathway to the same excited state. We demonstrate the concept experimentally by attosecond transition absorption spectroscopy in helium atoms. The quasi-forbidden transitions from the ground state 1s² to the weakly coupled doubly excited 2p3d and sp2,4− states are boosted by an order of magnitude. Enhancing single-photon-suppressed transitions can find widespread applicability, from spectral diagnostics of complex molecules in life and chemical sciences to precision spectroscopy of weak transitions in metastable atomic nuclei in the search for new physics.

We developed a theoretical method to study the high-order harmonic generation (HHG) from a gas target in an intense laser pulse. We obtain the microscopic HHG by solving the time-dependent Schrödinger equation with a single-active-electron approximation and the macroscopic HHG by summing over all the microscopic HHG with phase matching and self-absorption in two scales. One is in the laser wavelength scale, within which the laser peak intensity does not change while the propagation phase changes. Another is in the laser beam waist scale, within which the laser intensity and Gouy phase shift change. Taking the Ar atom as an example, we calculate the macroscopic HHG energy distribution and divergence. We also discuss the dependency of the macroscopic HHG on the gas pressure and gas jet position.