Selection rules for the high-order harmonic generation spectra

SELECTION RULES FOR THE HIGH-ORDER HARMONIC GENERATION SPECTRA 

HHG is a non-linear phenomena in which high multiplicities of a fundamental frequency are generated by irradiating a non-linear medium, usually composed of rare-gas atoms, with intense lasers [3,4]. As the wavelengflis obtained in HHG experiments have become shorter (e.g., 2.7 nm by Chang et al. [25] and below 2.5 nm by Schniirer et al. [26]), the 

It is straightforward to prove that [P2, Wf(r, t)] = 0. Thus, a complete set of QE states can be found, such that 

Note that the field-free Hamiltonian is invariant under the 00th order (i.e., continuous) rotation Coo = — P + p), where is arbitrary. Next, consider the following 00th order DS operator. 

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Selection rules for the high-order harmonic generation spectra 404... 

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