Effective Field and the Bloch-Siegert Shift at Bihromatic Excitation of Multiphoton EPR

TL;DR

This paper investigates multiphoton transitions in a two-level spin system under bichromatic excitation, deriving an effective Hamiltonian and analyzing the Bloch-Siegert shift's impact on resonance behavior and nutation amplitudes.

Contribution

It introduces a method to analyze multiphoton EPR transitions using an effective Hamiltonian and nonstationary nutation detection, highlighting the effects of inhomogeneous broadening.

Findings

Bloch-Siegert shifts reduce nutation amplitude but not frequency.

Multiphoton resonances are identified at specific combined frequencies.

Inhomogeneous broadening affects transition amplitudes, not frequencies.

Abstract

The dynamics of multiphoton transitions in a two-level spin system excited by transverse microwave and longitudinal RF fields with the frequencies w_{mw} and w_{rf}, respectively, is analyzed. The effective time-independent Hamiltonian describing the "dressed" spin states of the "spin + bichromatic field" system is obtained by using the Krylov-Bogoliubov-Mitropolsky averaging method. The direct detection of the time behavior of the spin system by the method of nonstationary nutations makes it possible to identify the multiphoton transitions for resonances w_{0} = w_{mw} + rw_{rf} (w_{0} is the central frequency of the EPR line, r = 1, 2), to measure the amplitudes of the effective fields of these transitions, and to determine the features generated by the inhomogeneous broadening of the EPR line. It is shown that the Bloch-Siegert shifts for multiphoton resonances at the inhomogeneous broadening of spectral lines reduce only the nutation amplitude but do not change their frequencies.