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The interaction of excited atoms and few-cycle laser pulses

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ABSTRACT

This work describes the first observations of the ionisation of neon in a metastable atomic state utilising a strong-field, few-cycle light pulse. We compare the observations to theoretical predictions based on the Ammosov-Delone-Krainov (ADK) theory and a solution to the time-dependent Schrödinger equation (TDSE). The TDSE provides better agreement with the experimental data than the ADK theory. We optically pump the target atomic species and measure the ionisation rate as the a function of different steady-state populations in the fine structure of the target state which shows significant ionisation rate dependence on populations of spin-polarised states. The physical mechanism for this effect is unknown.

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(a) Measurements of ionisation yield as a function of the angle that the quarter-wave plate makes relative to the polarisation axis defined by a linear polariser when using 640.24 nm optical pumping light. The intensity of the ionising laser is I = 9.2 × 1013 W/cm2. This intensity was chosen as it allows accurate use of ADK modelling of the ion yield for later analysis (see Fig. 1). The pump light is intended to pump the atom beam into an ensemble of different mj states, depending on the alignment of the fast axis of the quarter-wave plate with respect to a linear polariser. There is a significant ion yield difference between the Ne* beam being pumped with σ± circularly polarised light and being pumped with linearly polarised π light. This indicates an average ionisation potential difference between a spin-polarised atomic ensemble compared to a spin-averaged atomic ensemble. (b) Indicates the expected mj state fraction of the beam at different wave-plate angles. When pumped with π light, the state distribution for all remaining states are approximately 0.02. The modelling was performed for the experimental pump beam parameters by numerically solving the OBEs and is provided as a guide for the eye.
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f3: (a) Measurements of ionisation yield as a function of the angle that the quarter-wave plate makes relative to the polarisation axis defined by a linear polariser when using 640.24 nm optical pumping light. The intensity of the ionising laser is I = 9.2 × 1013 W/cm2. This intensity was chosen as it allows accurate use of ADK modelling of the ion yield for later analysis (see Fig. 1). The pump light is intended to pump the atom beam into an ensemble of different mj states, depending on the alignment of the fast axis of the quarter-wave plate with respect to a linear polariser. There is a significant ion yield difference between the Ne* beam being pumped with σ± circularly polarised light and being pumped with linearly polarised π light. This indicates an average ionisation potential difference between a spin-polarised atomic ensemble compared to a spin-averaged atomic ensemble. (b) Indicates the expected mj state fraction of the beam at different wave-plate angles. When pumped with π light, the state distribution for all remaining states are approximately 0.02. The modelling was performed for the experimental pump beam parameters by numerically solving the OBEs and is provided as a guide for the eye.

Mentions: Figure 3 shows the ion yield when rotating the quarter-wave plate of the optical pump light. The degree of circular polarisation of the optical pump light was measured by measuring the Stokes parameters utilising the classic method involving a linear polariser and a quarter-wave plate as described in ref. 32; in this case the normalised parameter S3/S0 describes the prevalence of σ+ circularly polarised light to σ− circularly polarised light. The average maximum absolute value for /S3/S0/max was measured to be 0.96 ± 0.02, i.e. the maximum degree of circular polarisation is 96% in this experimental setup. Figure 3(b) is a broad function around the point of full circular polarisation and such a change in the degree of circular polarisation will have a negligible effect on the optical pumping with a less than 1% change in the fully spin polarised state. A further consistency check for the ionisaton rate as a function of optical pumping was made by flipping the quarter wave plate so that the opposite handedness for the ionisation rate could be measured with the results consistent with what is presented in Fig. 3(a). Figure 3 shows that as pump light becomes more circularly polarised, there is a corresponding increase in the ionisation rate. There are a number of important observations that can be made about this measurement. The change in the ellipticity of the optical pumping beam changes the atomic state distribution of the Ne* atoms, and we clearly observe an ionisation dependence on the initial state of the Ne* system. The second observation is that the ionisation rate maximises for the fully spin polarised states compared to the mixed state case created by π polarisations which produces a mixed distribution between all mj substates. There also appears to be an asymmetry in the ionisation distribution. These are remarkable features and clearly demonstrates that the tunnel ionisation rate depends on the fine structure population of the excited state with maximum ionisation rates when the atoms are spin polarised.


The interaction of excited atoms and few-cycle laser pulses
(a) Measurements of ionisation yield as a function of the angle that the quarter-wave plate makes relative to the polarisation axis defined by a linear polariser when using 640.24 nm optical pumping light. The intensity of the ionising laser is I = 9.2 × 1013 W/cm2. This intensity was chosen as it allows accurate use of ADK modelling of the ion yield for later analysis (see Fig. 1). The pump light is intended to pump the atom beam into an ensemble of different mj states, depending on the alignment of the fast axis of the quarter-wave plate with respect to a linear polariser. There is a significant ion yield difference between the Ne* beam being pumped with σ± circularly polarised light and being pumped with linearly polarised π light. This indicates an average ionisation potential difference between a spin-polarised atomic ensemble compared to a spin-averaged atomic ensemble. (b) Indicates the expected mj state fraction of the beam at different wave-plate angles. When pumped with π light, the state distribution for all remaining states are approximately 0.02. The modelling was performed for the experimental pump beam parameters by numerically solving the OBEs and is provided as a guide for the eye.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5035976&req=5

f3: (a) Measurements of ionisation yield as a function of the angle that the quarter-wave plate makes relative to the polarisation axis defined by a linear polariser when using 640.24 nm optical pumping light. The intensity of the ionising laser is I = 9.2 × 1013 W/cm2. This intensity was chosen as it allows accurate use of ADK modelling of the ion yield for later analysis (see Fig. 1). The pump light is intended to pump the atom beam into an ensemble of different mj states, depending on the alignment of the fast axis of the quarter-wave plate with respect to a linear polariser. There is a significant ion yield difference between the Ne* beam being pumped with σ± circularly polarised light and being pumped with linearly polarised π light. This indicates an average ionisation potential difference between a spin-polarised atomic ensemble compared to a spin-averaged atomic ensemble. (b) Indicates the expected mj state fraction of the beam at different wave-plate angles. When pumped with π light, the state distribution for all remaining states are approximately 0.02. The modelling was performed for the experimental pump beam parameters by numerically solving the OBEs and is provided as a guide for the eye.
Mentions: Figure 3 shows the ion yield when rotating the quarter-wave plate of the optical pump light. The degree of circular polarisation of the optical pump light was measured by measuring the Stokes parameters utilising the classic method involving a linear polariser and a quarter-wave plate as described in ref. 32; in this case the normalised parameter S3/S0 describes the prevalence of σ+ circularly polarised light to σ− circularly polarised light. The average maximum absolute value for /S3/S0/max was measured to be 0.96 ± 0.02, i.e. the maximum degree of circular polarisation is 96% in this experimental setup. Figure 3(b) is a broad function around the point of full circular polarisation and such a change in the degree of circular polarisation will have a negligible effect on the optical pumping with a less than 1% change in the fully spin polarised state. A further consistency check for the ionisaton rate as a function of optical pumping was made by flipping the quarter wave plate so that the opposite handedness for the ionisation rate could be measured with the results consistent with what is presented in Fig. 3(a). Figure 3 shows that as pump light becomes more circularly polarised, there is a corresponding increase in the ionisation rate. There are a number of important observations that can be made about this measurement. The change in the ellipticity of the optical pumping beam changes the atomic state distribution of the Ne* atoms, and we clearly observe an ionisation dependence on the initial state of the Ne* system. The second observation is that the ionisation rate maximises for the fully spin polarised states compared to the mixed state case created by π polarisations which produces a mixed distribution between all mj substates. There also appears to be an asymmetry in the ionisation distribution. These are remarkable features and clearly demonstrates that the tunnel ionisation rate depends on the fine structure population of the excited state with maximum ionisation rates when the atoms are spin polarised.

View Article: PubMed Central - PubMed

ABSTRACT

This work describes the first observations of the ionisation of neon in a metastable atomic state utilising a strong-field, few-cycle light pulse. We compare the observations to theoretical predictions based on the Ammosov-Delone-Krainov (ADK) theory and a solution to the time-dependent Schrödinger equation (TDSE). The TDSE provides better agreement with the experimental data than the ADK theory. We optically pump the target atomic species and measure the ionisation rate as the a function of different steady-state populations in the fine structure of the target state which shows significant ionisation rate dependence on populations of spin-polarised states. The physical mechanism for this effect is unknown.

No MeSH data available.


Related in: MedlinePlus