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【正文】 me of the sample, the chormophor can be saturated ? Repetition rate (time between pulses) can be reduced (increased) by using a pulse picker. ? Acoustooptical pulse picker uses controlled diffraction of laser pulses on a grating generated by ultrasound Luminescence Upconversion 100 fs, 800 nm Sample Gate pulse delay Upconversion crystal Spectrometer SHG Luminescence Upconversion II Dt PL Pump Gate Upconversion process = gating PL (vis) Gate (NIR) Signal (UV) 1/ls = 1/lPL + 1/lG kS = kPL + kG IS ~ IG IPL(Dt) ?Intensity of the signal is proportional to intensity of PL at the moment of the gating pulse arrival. ? Resolution is determined by the gating pulse duration ? High repetition rate and power lasers are required ? Works well with photostable materials ? Limited delay range (mechanical delay 15 cm =1 ns) J. Shah, IEEE J. Quantum Electron. 24, p. 276, 1988 PumpProbe Experiments I Dt 0 1 2 Idea of the experiment Da 0?1 1?2 Before excitation After PumpProbe Experiments II ? PPE enable one to trace the relaxation dynamics with sub100 fs resolution ? Types of the data generated by PPE: timeresolved absorption spectra and absorption transients at a certain wavelength. ? Numerous binations of pump and probe beams are possible (UV pump + visible probe, UVpump+continuum probe, etc.) ? High pump intensities are required in order to produce noticeable change in the optical absorption of the sample (GW/cm2 – TW/cm2) (Ti:Sapphire amplifiers are generally required) ? Interpretation of the data is sometimes plicated PumpProbe Experiments III 100 fs, 800 nm Probe pulse delay Spectrometer Continuum generator SHG Lockin amplifier ? Detects 105 transmission change ? PPE spectra can be chirpcorrected during the experiment ? Use of continuum as a probe enables one to cover the entire visible and NIR ranges . Klimov and . McBranch, . 23, p. 277, 1998 Semiconductor Quantum Dots 2. 01. 51. 00. 50. 0ad3. 23. 02. 82. 62. 42. 22. 01. 8 P h o t o n E n e rg y ( e V )R = 4 . 1 n m1 . 7 n m1 . 2 n m1S ( e ) 1S3 / 2( h)1S ( e ) 2S3 / 2( h)1P ( e ) 1P3 / 2( h)1S ( e ) 3S1 / 2( h) 106 PL Intensity (arb. units) P h o t o n E n e rg y ( e V )R = 4 . 1 n m1 . 7 n m1 . 2 n mTransient Absorption Spectrscopy of CdSe Quantum Dots 40x103200 2 0Da (arb. units)2 . 62 . 42 . 22 . 01 . 8P h o t o n E n e r g y ( e V )C d S e N C 39。 s (300K )1 S ( e ) 1 S3 / 2( h )1 S ( e ) 2 S3 / 2( h )1 P ( e ) 1 P3 / 2( h )1 S ( e ) 3 S1 / 2( h )D t = 2 0 0 f sR = 4 . 1 n m Da d Delay time (ps) t (567 nm) =540 fs t (644 nm) =530 fs 400 fs CdSe NC39。s (R = nm) 644 nm (1S) 567 nm (1P) (a) Klimov . and McBranch D. W., . 80, p. 4028, 1998
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