R-matrix Floquet Approach Research Articles

On the importance of inner-shell processes in two-photon ionisation of Ne+

Using the R-matrix Floquet approach and the time dependent R-matrix approach, we investigate two-photon ionisation of Ne+ in its ground state 1s22s22p5 2Po, irradiated by laser light with intensity 1014 W cm−2. We show that the atomic structure has a huge influence on the ionisation rates as excitations from the 2s shell strongly affect the photo-ionisation process.

On the importance of inner-shell processes in two-photon ionization of Ne+

We have investigated two-photon ionization of ground-state Ne+ using the time-independent R-matrix Floquet approach and the time-dependent R-matrix approach. The R-matrix Floquet approach enables the detailed study of the influence of atomic structure on the magnitude of the ionization rates. It is found that excitation processes involving the inner 2s electron substantially enhance the two-photon ionization rates of Ne+. For photon energies above 37.5 eV, emission of the 2s electron becomes significant as demonstrated by the large probability that the residual Ne2+ ion is left in the 2s2p5 configuration. Time-dependent calculations of the ionization probability as a function of pulse length correspond well with the R-matrix Floquet ionization rates. However, the ionization probability is enhanced by single-photon ionization during the turn-on and turn-off of the laser pulse. In addition, the laser pulse leads to the population and depopulation of autoionizing states above the Ne2+ ground state during the pulse, and the final population left in these states strongly depends on pulse length.

Two-photon emission of a 2p electron from Ne+ and consequences for sequential double ionization of Ne

Using the R-matrix Floquet approach, we investigate two-photon emission of a 2p electron from the 1s22s22p5 ground-state of Ne+ irradiated by laser light with photon energies between 25 eV and 40 eV and an intensity of 1014 W cm−2. We obtain 2p emission rates for Ne+ initially in the M = 0 and M = 1 levels. Two-photon emission of the 2p electron from the M = 0 level leaves Ne2+ predominantly in 2p41De, whereas two-photon emission of the 2p electron from the M = 1 level leaves Ne2+ predominantly in 2p43Pe. In sequential double ionization of Ne, the first ionization step thus has a noticeable influence on the second ionization step. The present rates differ significantly from ionization rates deduced from experiment. The main reason for this is probably that two-photon emission of a 2s electron is the dominant ionization process at 38.4 eV.

Resonance effects on the competition between multiphoton emission of inner-shell and outer-shell electrons

Using the R-matrix Floquet approach, we have investigated ionization of the 1s2s 1S and 1s3s 1S states of He subjected to VUV radiation, following the absorption of two, three and four photons, with a focus on the influence of resonances on the state that the residual He+ ion is left in. For the 1s2s 1S state, we find that final-state 3ℓnℓ′ resonances enhance the relative probability that He+ is left in the n = 2 states. Intermediate 2ℓnℓ′ resonances strongly enhance ionization following three-photon and four-photon absorption. The 2ℓ2ℓ′ resonances enhance in particular the probability that He+ ion is left in n = 2, while higher 2ℓnℓ′ enhance in particular the probability that He+ is left in n = 3, since photoionization of the 2ℓnℓ′ resonances is dominated by emission of the inner 2ℓ electron. For the 1s3s 1S state, we find a stronger influence of the final-state 3ℓnℓ′ resonances on the relative probability that after two-photon absorption He+ is left in n = 2.

Competition between multi-photon emission of the 1s and the 2s electron from He 1s2s 1S

Using the R-matrix Floquet approach, we have investigated multiphoton ionization of the 1s2s 1S state of He. We compare emission rates for the 1s and 2s electrons after absorption of N photons, where N is the minimum number of photons needed to emit the 1s electron. For N = 1 and 2, we find that emission of the 1s electron dominates, while for N = 4 emission of the 2s electron dominates. For N = 3, both processes are of comparable magnitude. In the absence of resonances, emission of the 1s electron typically becomes less likely by a factor of 5 for each additional photon absorbed. However, for N = 4, emission of the 1s electron is strongly enhanced due to the intermediate 2s2p 1P resonance.

Single- and two-photon ionization of Sr

Using the R-matrix Floquet approach we have calculated single- and two-photon ionization cross sections for Sr within a model potential approach. The frequency ranges investigated range from 0.21 to 0.27 au and from 0.105 to 0.158 au. The results are in reasonable agreement with previous experimental and theoretical work. The two-photon cross sections are dominated by the 5s6p 1Po resonance. For the m = 0 level of this resonance, we obtain photoionization cross sections ranging typically between 5 and 20 Mb. A comparison with results for Ca shows good similarity for single-photon ionization, but significantly less agreement for two-photon ionization.

Two- and three-photon ionization of He between 1013 and 1014 W cm−2

We have applied the R-matrix Floquet approach to study two- and three-photon ionization of He for intensities between 1013 and 1014 W cm−2. The non-perturbative influence of the laser field manifests itself in a number of ways: resonance positions shift as a function of intensity, and the widths of both bound-state and autoionizing resonances increase due to their increased photoionization. We demonstrate that even at relatively modest intensities the widths of some low-lying autoionizing states are dominated by photoionization.

Single- and multiphoton detachment of K−

Single- and multiphoton detachment rates have been calculated for K− usingthe R-matrixFloquet approach. Single-photon detachment rates, obtained at alaser field peak intensity of 109 W cm−2,are discussed and compared with other theoretical work. Two-photondetachment rates at the same intensity have also been obtained,and similarities with results from earlier calculations for Li− andNa−are discussed. Three-photon rates are also presented at this laser intensity,and are compared and contrasted with those arising in the single-photoncase, since both involve resonance structure with 1Posymmetry. The influence of resonances such as the 5s2 1Sedoubly excited state and excitations of the residual atom are also considered.

Multiphotonionization processes in Ar7+

We have studied two- and three-photon ionization of the 3s and3p (mL = 0) states in Ar7+ at an intensity of1015 W cm-2. The 4f state appears prominently in the3s two-photon ionization spectrum due to light-induced continuumstructure. The three-photon ionization rate for the 3p state isstrongly influenced by the 4s resonance after absorption of asingle photon and the np and nf Rydberg series after absorptionof two photons. The calculations demonstrate the efficiency ofthe R-matrix Floquet approach for highly charged ions in x-raylaser fields.

Single andmultiphoton detachment of Na-

Single and multiphoton detachment rates have been calculated for Na-using the R-matrix Floquet approach. In the low-intensity limit theresults are in agreement withother recent calculations of the totalphotodetachment cross section. At intensities of 1010 W cm-2 two-photon detachment rates are presented and similaritieswith earlier calculations in Li- are discussed. Three-photon rates arealso presented at this intensity and are compared and contrasted with thesingle-photon case since both involve resonance structure with 1Po symmetry. The influence of resonances such as the 4s21Se doubly excited state and excitations of the residualatom are also considered.

Configuration-interaction effects in and intensitydependence of F- multiphoton detachment

Two- and three-photon detachment rates have been obtained forF- using several expansions in the R-matrix Floquetapproach. These rates are compared with other theoretical andexperimental results. The use of Hartree-Fock wavefunctions forthe ground state of F with addition of continuum electrons doesnot lead to agreement with experiment for two- andthree-photon detachment. By adding correlation terms, agreementwith experiment and other theoretical results is improvedconsiderably, demonstrating the importance of electroncorrelation effects. However, convergence with respect to thewavefunction expansion cannot be established. We also studythe intensity dependence of multiphoton detachment rates forF- at the Nd-YAG frequency. Due to the ponderomotive shiftthe three-photon detachment channel closes at an intensity of8.5×1011 W cm-2 and the influence of this channelclosure on the multiphoton detachment peaks is illustrated bydetermining the heights of the excess-photon peaks obtainedusing a Gaussian laser pulse.

R-matrix Floquet theory of multiphoton processes: VI. The negative halide ions

The R-matrix Floquet approach is applied to study the negative and ions in a light field. Detachment rates are obtained for detachment processes involving up to three photons. The results obtained in the present approach are compared to other experimental and theoretical results. For two- and three-photon processes reasonable agreement with other calculations has been found, while for two-photon detachment the results agree with the experimental cross sections. The three-photon results are in less good agreement with experiment although the larger error bars make accurate comparisons more difficult. The changes in the detachment behaviour for these ions are compared to each other as well as to the detachment behaviour of .

R-matrix Floquet theory of multiphoton processes: VII. A two-colour approach

The R-matrix Floquet approach has been extended to treat multiphoton processes involving two incommensurate laser frequencies. The extensions to the R-matrix Floquet theory are discussed and the approach is illustrated with an example of light-induced continuum structure in He, linking the 1 ground state with the 1s2s state. The results obtained for this example are compared with model calculations and good agreement is found, if the q parameter is suitably chosen. A further example is provided by linking the ground state of He and the 1s2s state with the autoionizing state by two-photon processes. The differences due to the autoionizing state are illustrated by showing the changes in the behaviour of the and 1s2s states. The first example demonstrates that the code is able to describe light-induced continuum structure, while the second example demonstrates that the code is capable of describing multi-electron excitations in two laser fields with different frequencies. It is also demonstrated that for more extensive calculations, it is necessary to develop new approaches which limit the amount of memory required.