meyer_2008 ========== M\. Meyer, D\. Cubaynes, D\. Glijer, J\. Dardis, P\. Hayden, P\. Hough, V\. Richardson, E\. T\. Kennedy, J\. T\. Costello, P\. Radcliffe, S\. Düsterer, A\. Azima, W\. B\. Li, H\. Redlin, J\. Feldhaus, R\. Taïeb, A\. Maquet, A\. N\. Grum\-Grzhimailo, E\. V\. Gryzlova and S\. I\. Strakhova, "Polarization Control in Two\-Color Above\-Threshold Ionization of Atomic Helium", Phys\. Rev\. Lett\. 101, 193002 \(2008\) `Source record `__ · :doc:`Catalogue ` · :doc:`Review register ` DOI: `10\.1103/PhysRevLett\.101\.193002 `__ **Source kind:** measurement **Recorded technique:** Magnetic\-bottle photoelectron spectroscopy of helium \(effusive jet\) ionised by the FLASH free\-electron laser \(single bunch, 5 Hz, 10\-20 fs, about 20 uJ, 25\.5 nm = 48\.6 eV or 13\.7 nm = 90\.5 eV, bandwidth 0\.5\-1 %, focus about 30 um\) in collinear overlap with a synchronised picosecond optical dressing laser \(523 nm Nd\:YLF second harmonic, about 500 uJ, 12 ps; or 800 nm Ti\:sapphire, up to 2 mJ, 4 ps; focus about 50 um\), both linearly polarised \(about 99 %\), the relative angle theta set by a half\-wave plate; \-60 V retardation, so only the high\-energy sidebands are recorded\. Theory\: time\-dependent second\-order perturbation theory \(single active electron, local potential\) and the soft\-photon approximation \(pp\. 193002\-1 to \-3\)\. **Materials in structured entries:** None recorded. **Dataset quantities:** None recorded. **Contents:** 0 datasets; 0 formulas. Curation notes -------------- Context only, no datasets\. Identity checked on the first page \(PDF p\. 1\)\: Meyer et al\., Phys\. Rev\. Lett\. 101, 193002 \(2008\), week ending 7 November 2008, four pages 193002\-1 to \-4 \(PDF p\. n = 193002\-n\)\. Every number below was read on the PDF's text layer and the figure images\. WHAT THE PAPER MEASURES\. Two\-colour above\-threshold ionisation of He 1s\: one XUV photon ionises \(the main line, He\+ 1s\) and the electron exchanges n optical photons in the continuum \(sidebands SBn\)\. Only bound\-continuum and continuum\-continuum steps\: no bound two\-photon resonance and no resonant intermediate level\. The quantity measured is the dependence of the sideband yield on theta, the angle between the two linear polarisations, in arbitrary units \(Figs\. 3 and 4 are normalised to 1 at the maximum; Fig\. 2's spectra are in arbitrary units\)\. The numbers \(pp\. 193002\-2 to \-4\)\: \- low dressing field, about 8 x 10^10 W/cm2 \(800 nm, 13\.7 nm XUV; one sideband\)\: sigma\(theta\) proportional to 3 S2 \+ \(5 S0 \+ S2\) cos^2 theta \(Eq\. 1\), S\_i = \|T\_i\|^2 the squared second\-order radial integrals to the s \(i = 0\) and d \(i = 2\) waves; measured contrast sigma\_max / sigma\_min = 3\.4 \+\- 0\.5 \(theory 3\), whence S0\:S2 = 1\.25 \+\- 0\.3, against 0\.98 \(second\-order perturbation theory\) and 1\.0 \(soft\-photon\); at theta = 0 the s\:d yield ratio 5 S0 \: 4 S2 'of 1\.5'\. Recomputed\: contrast \(5 S0 \+ 4 S2\) / \(3 S2\) = 3\.4 gives S0\:S2 = 1\.24; S0\:S2 = 0\.98 gives 2\.97; 5 x 1\.25 / 4 = 1\.56; \- 25\.5 nm \(48\.6 eV\) with 523 nm dressing\: S0\:S2 'about 0\.9 \+\- 0\.3', simulation 0\.85; \- soft\-photon approximation \(Eqs\. 2\-3\)\: sigma^\(n\) proportional to a\_n \+ b\_n cos^2 theta, b\_n / a\_n = 2n at low field, a contrast of 2n \+ 1; \- high field, about 6 x 10^11 W/cm2 \(800 nm, 13\.7 nm; up to three sidebands, Fig\. 2\)\: SB1 contrast 1\.5 \+\- 0\.3 \(soft\-photon 1\.47\); SB2 about 3\.3 \(calculation about 3\.5\), below the limit 5; \- photon energies recomputed\: 1239\.84 / 25\.5 = 48\.62 eV, / 13\.7 = 90\.50 eV; the 90\.5 eV main line at about 66 eV kinetic energy in Fig\. 2 is 90\.5 \- 24\.59 \(He's ionisation energy\)\. The uncertainty's level is not stated\. NOT TAKEN \(no quantity; declined, TC\-1011\)\: the S0\:S2 ratios, the contrasts and the s\:d yield ratio are two\-colour sideband ratios\. No absolute quantity is printed\: no two\-photon \(XUV \+ optical\) cross section, no absolute amplitude T\_i or phase, no generalised cross section; the S\_i appear only as ratios\. The figures carry no absolute scale, so nothing would be digitised\. FOR OEP 8 AND OEP 6\. Not a test of either model's bound two\-photon physics\: the paper probes free\-free transitions far above threshold \(about 60\-66 eV electrons at 90\.5 eV\)\. One remark bears on excited\-level photoionisation \(OEP 8's Gamma^PI\)\: it quotes Johansson et al\. \(Eur\. Phys\. J\. D 22, 3 \(2003\), not in the library\) for resonant two\-colour ionisation through He 1snp 1P \(n = 2, 3\), where the d\-wave cross section is 'more than 10 times higher' than the s\-wave near threshold \(p\. 193002\-3\); a partial\-wave ratio, not an absolute cross section\. Two\-colour references the paper cites that the library does not hold\: R\. Taïeb et al\., Phys\. Rev\. A 62, 013402 \(2000\) \(the second\-order radial integrals of Eq\. 1\); A\. Maquet and R\. Taïeb, J\. Mod\. Opt\. 54, 1847 \(2007\) \(the soft\-photon approximation\); P\. O'Keeffe et al\., Phys\. Rev\. A 69, 051401\(R\) \(2004\); O\. Guyétand et al\., J\. Phys\. B 38, L357 \(2005\) and 41, 051002 \(2008\); E\. S\. Toma et al\., Phys\. Rev\. A 62, 061801 \(2000\); T\. E\. Glover et al\., Phys\. Rev\. Lett\. 76, 2468 \(1996\)\. **Recorded bibliographic identity status:** VALIDATED. This concerns identification of the source; it does not establish the scientific correctness of its contents. Datasets -------- No datasets are recorded. Formulas -------- No formulas are recorded. Corrections ----------- No dataset errata are recorded in this source record. Formula corrections, where recorded, appear with their formulas above. Curation notes may also discuss corrections or unresolved interpretation. Consult the :doc:`review register ` for the checks and decisions.