walker_1994 =========== B\. Walker, B\. Sheehy, L\. F\. DiMauro, P\. Agostini, K\. J\. Schafer and K\. C\. Kulander, "Precision measurement of strong field double ionization of helium", Phys\. Rev\. Lett\. 73, 1227 \(1994\) `Source record `__ · :doc:`Catalogue ` · :doc:`Review register ` DOI: `10\.1103/PhysRevLett\.73\.1227 `__ **Source kind:** measurement **Recorded technique:** He\+ and He2\+ yields against peak intensity over twelve orders of magnitude of counting range, and photoelectron spectra\: 780 nm, 160 \+\- 30 fs pulses, 1 kHz, up to 1\.3 mJ \(pulse\-to\-pulse fluctuation \< 1\.7 % over 10^6 shots\), linear polarisation \(extinction ratio \> 100\), 10 cm focal\-length doublet into an ultrahigh vacuum \(10^\-10 torr\); 99\.999 % helium scrubbed to \< 0\.1 ppm of O2, H2, H2O, CO2 and hydrocarbons; 30 cm time\-of\-flight mass spectrometer \(m/dm \> 300, H2\+ and He2\+ separated\), 1 ns binning of discriminated events at \<= 0\.25 events per shot; \>= 60 000 shots per point, five scans with three spot sizes \(p\. 2\)\. Compared with single\-active\-electron \(SAE\) TDSE yields \(Schafer et al\. 1993\) and ADK \(pp\. 2\-3\)\. **Materials in structured entries:** None recorded. **Dataset quantities:** None recorded. **Contents:** 0 datasets; 0 formulas. Curation notes -------------- Journal pp\. 1227\-1230 = pdf pages 1\-4\. Identity checked against pdf p\. 1 \(the library flags a year hint of 2011, the download date; the page prints 29 August 1994\)\. WHY NO YIELDS ARE TAKEN\. The yields are in Fig\. 1 only \(He\+ and He2\+ ion signal against intensity, 10^14 to 10^16 W/cm^2, with the SAE \(solid\) and ADK \(dashed\) He\+ curves, the NS He2\+ model curve \(dotted\) and the calculated sequential He2\+ yield\), and the He2\+\(NS\)/He\+ ratio in Fig\. 3 \(with 1 standard deviation error bars\); not digitised\. The ion signal is not absolute\. PULSE\. '160 \+\- 30 fsec, 780 nm' \(text p\. 2, and the abstract's '160 fs'\), but the Fig\. 1 caption says '100 fsec'\: an inconsistency of the paper; the text's 160 fs is the stated measurement\. CALIBRATION \(p\. 2\)\. 'The laser intensity is calibrated by measuring the short pulse resonant photoelectron spectrum of xenon as a function of pulse energy and concurrently recording the xenon and helium ion yields\. The calibration is corroborated by a spot size measurement and accurate to approximately 30%\.' In Figs\. 1 and 3 'The measured intensities are multiplied by 1\.15' \(captions\), i\.e\. the plotted intensity scale is the measured one times 1\.15 \(the paper does not say why; the factor lies within the stated 30 %\)\. e/m detection sensitivity 1\:1\.25 \(He\+\:He2\+\), 'based on saturated yields'\. TEXT VALUES \(p\. 2\-4; not taken as data, since no quantity holds a saturation intensity or a yield ratio\)\: 'The He\+ yield increases nonperturbatively up to a measured saturation intensity of 8 x 10^14 W/cm^2\. Beyond this point the yield increases as I^\(3/2\)'\. He2\+\: NS production at \(1\.5\-8\) x 10^14 W/cm^2, saturated regime \(0\.8\-3\) x 10^15 W/cm^2, sequential production above 3 PW/cm^2 and saturation at 8 PW/cm^2\. 'the ratio exhibits a gentle slope of only I^1\.3\. The ratio is constant \(0\.0020 \[3\]\) from about 5 PW/cm^2 until the sequential production of He2\+ becomes significant' \(p\. 3, as printed; Fig\. 3 shows the plateau from about 0\.5 PW/cm^2 and sequential production is said to start at 3 PW/cm^2, so '5 PW' reads as a slip for 0\.5 PW\); the conclusion\: 'a saturated value of 0\.0020' \(p\. 4\)\. The ion curves vary by 7 orders of magnitude between 0\.15 and 5\.0 PW/cm^2 \(p\. 3\)\. SAE agrees with the He\+ yield over the whole range, ADK falls below it at low intensity, 'the multiphoton contribution becoming increasingly important below 0\.5 PW/cm^2' \(p\. 2\); the tunnelling fraction of the SAE rate rises from 0\.05 to 1 over the range \(p\. 3\)\. The rescattering threshold 3\.2 U\_p \>= 54\.4 eV would be 0\.3 PW/cm^2 \(0\.23 allowing excitation to 41 eV and barrier suppression\); the data go down to 0\.14 PW/cm^2 without a threshold \(p\. 3\); U\_p = 5\.7 eV at 10^14 W/cm^2 and 780 nm \(ref\. 10\)\. This paper's He\+ saturation is the reference against which larochelle\_1998 calibrates its intensities \(ADK's He saturation\)\. **Recorded bibliographic identity status:** VALIDATED\_WITH\_WARNINGS. 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.