Experimental and theoretical study of three-photon ionization of He($1s2p{}^3{P}^o$)

A joint experimental and theoretical study of three-photon ionization of the $1s2p{}^3{P}^o$($M_L=0,\pm 1$) states of helium is presented. The ion yield is recorded in the 690–730 nm wavelength range for different laser pulse energies, using an excited helium beam produced by photodetachment of helium negative ions. Two series of asymmetric peaks due to two-photon resonances with $1snp$ and $1snf$ Rydberg states are observed. In one series, the peaks have tails towards higher frequencies, while in the other series the tails change direction for higher Rydberg states. An effective Hamiltonian is built in the dressed state picture, and a numerical model simulating the traversal of the helium atom across the laser pulse is developed. The simulated and observed ion yields are in good qualitative agreement. The observed behavior is shown to result from the contributions of two different resonantly enhanced multiphoton ionization processes, depending on the magnetic quantum number $M_L$ of the initial state. The asymmetry reversal is explained by the strong $1s2p$–$1s3s$ dynamic Stark mixing for $M_L=0$.

Figure 1

Schematic energy level diagram. The dotted arrows correspond to ($1+1+1$) REMPI, allowed only for $M_L=0$, while the full arrows represent a ($2+1$) REMPI, allowed for both $M_L=0$ and $M_L=\pm 1$. The interactions $V_{ij}^{\left(2\right)}$ are two-photon dipole matrix elements, while ${\Gamma }_{i\varepsilon \ell }$ are partial ionization rates.

Figure 2

Experimental setup. Cs: cesium vapor cell; PD: planar deflector; FC: Faraday cup; CD: cylindrical deflector; IR: interaction region; Q: quadrupolar deflector; MCP: multichannel plates. The double arrow indicates a convergent lens. The laser beam propagates along the $z$ direction and its polarization is along the $y$ axis.

Figure 3

Real part of the RMF quasienergies for $M_L=\pm 1$ and dressed Rydberg states from $n=8$ to $n=14$, as a function of the laser angular frequency at a fixed intensity of $3.6\times {10}^{10}$ W/cm${}^2$. The dashed line indicates the zero-field energy of the $1s2p$ state and the thick dot-dashed line in the upper right corner is the two-photon ionization threshold.

Figure 4

Real parts of the RMF quasienergies for $M_L=0$ and dressed Rydberg states from $n=8$ to $n=16$, as a function of the laser angular frequency at a fixed intensity of $3.6\times {10}^{10}$ W/cm${}^2$. The horizontal dashed line indicates the zero-field energy of the $1s2p$ state, while the oblique dashed line represents the zero-field energy of the $1s3s$ state shifted down by $\omega$. The thick dot-dashed line in the upper right corner is the two-photon ionization threshold.

Figure 5

Real part of the EH quasienergies for $M_L=\pm 1$ as a function of the laser angular frequency at a fixed intensity of $3.6\times {10}^{10}$ W/cm${}^2$. For each pair of Rydberg states, the lower line corresponds to $1snp$, and the upper line to $1snf$, with $6\le n\le 22$. The horizontal dashed line corresponds to the field-free energy of the $1s2p$ state. The thick dot-dashed line in the upper right corner is the two-photon ionization threshold. The inset is a magnified view of the crossing highlighted in the small box.

Figure 6

Real part of the EH quasienergies for $M_L=0$ as a function of the laser angular frequency at a fixed intensity of $3.6\times {10}^{10}$ W/cm${}^2$. For each pair of Rydberg states, the lower line corresponds to $1snp$, and the upper line to $1snf$, with $6\le n\le 22$. The horizontal dashed line corresponds to the field-free energy of the $1s2p$ state, while the oblique dashed line corresponds to the energy of the $1s3s$ state shifted down by $\omega$. The thick dot-dashed line in the upper right corner is the two-photon ionization threshold. The inset is a magnified view of the crossing highlighted in the small box.

Figure 7

Total ionization rates for $M_L=\pm 1$, for $n=7$ up to $n=16$, as a function of the laser angular frequency at a fixed intensity of $3.6\times {10}^{10}$ W/cm${}^2$. The inset is a magnified view of the narrow region highlighted by the box.

Figure 8

Total ionization rates for $M_L=0$, for $n=7$ up to $n=16$, as a function of the laser angular frequency at a fixed intensity of $3.6\times {10}^{10}$ W/cm${}^2$. The inset is a magnified view of the narrow region highlighted by the box.

Figure 9

Simulated ionization spectrum for a laser pulse energy of 6 mJ. The shaded curve is the contribution from $M_L=\pm 1$ states and the full line is the weighted sum of the contributions from $M_L=0$ and $M_L=\pm 1$ states.

Figure 10

Experimental ionization spectrum for a laser pulse energy of 6 mJ. The vertical dashed line indicates the $1s2p$-$1s3s$ resonance frequency ${\omega }_r$.

Figure 11

Simulated ionization spectra for 8, 6, and 2 mJ laser pulses.

Figure 12

Experimental ionization spectra for 8, 6, and 2 mJ laser pulses. The vertical dashed line indicates the $1s2p$-$1s3s$ resonance frequency ${\omega }_r$.