Absolute Optical Cross Sections Research Articles

UV-Vis Ratiometric Resonance Synchronous Spectroscopy for Determination of Nanoparticle and Molecular Optical Cross Sections.

Demonstrated herein is a UV-vis Ratiometric Resonance Synchronous Spectroscopic (R2S2, pronounced as "R-two-S-two" for simplicity) technique where the R2S2 spectrum is obtained by dividing the resonance synchronous spectrum of a NP-containing solution by the solvent resonance synchronous spectrum. Combined with conventional UV-vis measurements, this R2S2 method enables experimental quantification of the absolute optical cross sections for a wide range of molecular and nanoparticle (NP) materials that range optically from pure photon absorbers or scatterers to simultaneous photon absorbers and scatterers, simultaneous photon absorbers and emitters, and all the way to simultaneous photon absorbers, scatterers, and emitters in the UV-vis wavelength region. Example applications of this R2S2 method were demonstrated for quantifying the Rayleigh scattering cross sections of solvents including water and toluene, absorption and resonance light scattering cross sections for plasmonic gold nanoparticles, and absorption, scattering, and on-resonance fluorescence cross sections for semiconductor quantum dots (Qdots). On-resonance fluorescence quantum yields were quantified for the model molecular fluorophore Eosin Y and fluorescent Qdots CdSe and CdSe/ZnS. The insights and methodology presented in this work should be of broad significance in physical and biological science research that involves photon/matter interactions.

Absolute Optical Cross Section of Individual Fluorescent Molecules

Besides having revealed transitions masked in the bulk, optical spectroscopy of single molecules has enabled the observation of elementary molecular quantum events, such as the absorption and emission of a photon. Single photon transitions are quantified by a cross section that can be regarded as the photon capture area of a molecule. Therefore, absolute optical cross sections can readily be measured in the bulk from the decrease (by absorption) or the gain (by stimulated emission) of the number of photons in a beam. Unfortunately, this method is not applicable to individual molecules, because the resulting change in intensity is negligible. If the fluorescence quantum efficiency Qem is known, the molecular absorption cross section can be calculated from the fluorescence emission. However, this method re-introduces the hallmark of ensemble averaging, because Qem is known only from the bulk. As a result, the absolute photon capture area of a specific molecule has remained elusive. Here we report on the measurement of absolute cross sections of stimulated emission at room temperature. No a priori information about the individual molecule is needed. In addition, we demonstrate the instant control of a molecule.s excited state by light and thus, as a matter of fact, also the viability of pump–probe experiments with individual molecules at room temperature. Molecular optical absorption usually takes place from the vibrationally relaxed ground state S0 to a hot Franck–Condon state S1* of the first excited singlet state (Figure 1a). After vibrational relaxation, which occurs on a (sub)picosecond time scale, organic fluorophores emit within a few nanoseconds with a probability given by the fluorescence quantum yield. Alternatively, the S1!S0 transition can be enforced by light, that is by stimulated emission. Stimulated emission has been employed to probe the temporal evolution of the excited state in a pump–probe fashion, as well as to alter the fluorescence lifetime and anisotropy of organic fluorophores in solution. Furthermore, stimulated emission depletion (STED) of the excited state has been introduced in fluorescence microscopy to break the diffraction resolution barrier, but has recently also been used to measure cross sections in the bulk. We attained single molecule sensitivity with a scanning confocal setup fed by two synchronized pulse trains: a first pulse at a wavelength of l= 565 nm for excitation was followed by a red-shifted pulse at l= 778 nm for STED, both adapted to the spectrum (Figure 1b) of the xanthene dye JA 26 (1). Efficient STED demands a STED pulse duration t

The electronic states of cyclopropane studied by VUV absorption and electron energy-loss spectroscopies

Absolute optical (VUV) absorption cross sections for cyclopropane have been measured from 5.0 to 11.2 and 20-40 eV using synchrotron radiation. Also, electron energy-loss (EEL) spectra have been obtained using incident electrons of (a) 150 eV energy scattered through small angles (energy loss 5.0-15 eV) and (b) near-threshold energies scattered through large angles (energy loss 0-10.5 eV). Taken together these confirm that the low-lying excited electronic states of cyclopropane are of Rydberg type and, although spectral bands are diffuse, a known Rydberg series has been extended. Recent computations (Galasso V 1996 Chem. Phys. 206 289) appear to give a good account of the experimental spectrum from threshold to about 11 eV, but these must be extended if valence-excited states are to be characterized. Particular attention has been directed at the evaluation of absolute optical cross sections. These are now believed to be established over the energy ranges 5-15 and 20-40 eV. In the gap region (15-20 eV) second-order radiation may affect the optical measurements. From consideration of second-order effects, and comparison of the present studies with earlier measurements, we propose a best-estimate cross section in this energy region also.

Electron impact excitation of the 3p(5P) state of atomic oxygen

The 157.6 nm output from a fluorine excimer laser is focused in a vacuum chamber containing O2 gas at a pressure of 20 mTorr. Laser photodissociation of the O2 target gas produces 2p4(3P) and 2p4(1D) oxygen atoms with unit quantum efficiency. A low-energy electron beam is crossed with the laser beam to produce 3p(5P) excited states of OI via electron impact excitation of the 2p4(3P) ground state. Intensity measurements of the 777.4 nm radiation, corresponding to the 3p(5P)–3s(5S) transition, are used to calculate the excitation cross section of the 3p(5P) state of atomic oxygen. Absolute optical cross sections are reported for a range of incident electron energies less than 18 eV.

Electron Excitation of the First Positive Bands of N_2 and of the First Negative and Meinel Bands of N_2^+*

Absolute optical cross sections for the excitation from the X2∑g+ ground state of N2 by electron impact have been measured for 39 N2 first positive bands (B3Πg–A3∑u+), 12 N2+ first negative bands (B2∑u–X2∑g+), and 10 N2+ Meinel bands (A2Πu–X2∑g+). Relative excitation functions are presented for N2 first negative bands with v′ values from 0 through 9 for 0 to 50 eV, for the (0,0) and (1,0) N2+ first negative bands for 0 to 450 eV, and for the (3,0) and (4,1) N2+ Meinel bands for 0 to 450 eV. The first-positive excitation functions exhibit sharp peaks at 10–12 eV and at 14–15 eV and exhibit an S-shaped change in slope about midway between onset and the maximum of the first sharp peak. The first negative and Meinel excitation functions have broad maxima at about 120 eV and 100 eV.

Absolute optical cross sections of cells and chloroplasts

Absolute optical cross sections in square microns of Escherichia coli cells, spinach chloroplasts, and yeast cells were measured and compared with theoretical predictions for simple particle models. The area of these cross sections indicates the absolute amount of light per particle which is scattered at all angles or absorbed. The cross sections are obtained from transmissions of counted suspensions as measured with a special single-beam spectrophotometer. At a wavelength of 500 mμ, the average yeast cell is found to scatter 310% of the light striking its geometrical cross section. The spinach chloroplast scatters and absorbs a total of about 150% of the light that strikes it, while an E. coli cell scatters about 33%. Mie and anomalous diffraction equations were used to calculate theoretical cross sections from measured values of particle size, shape, refractive index, and gross internal structure. Absolute suspension transmissions were then calculated from these cross sections and the values of particle count. The resulting theoretical predictions quantitatively agree with observations. This agreement substantially establishes light-scattering theory as a reliable tool for studies of large biological particles. The same basic theory indicates that scattering by such particles is accomplished mainly by interference and diffraction, not by less potent mechanisms similar to surface reflection or Rayleigh scattering.