E. Jannitti

Amorphous silicon/silicon carbide photodiodes with excellent sensitivity and selectivity in the vacuum ultraviolet spectrum

An innovative family of thin‐film photodetectors optimized for the ultraviolet (UV) spectrum is presented here. The devices are made of hydrogenated amorphous silicon (a‐Si:H) and silicon carbide (a‐SiC:H) on glass substrates. At room temperature, the photodetectors exhibit values of quantum efficiency of 21% in the vacuum UV and 0.08% at 750 nm, without external voltage. The great advantage of this technology lies in the possibility to produce low‐cost, large‐area arrays of photodetectors on glass or flexible substrates. All these features candidate the a‐Si/SiC:H photodetectors as possible, concurrent to specialized commercial devices.

Toroidal grating obtained on an elastic substrate

The performance of a grating that is elastically deformed from a sphere into a toroid is that expected from ray traces assuming an ideal toroidal surface. Thus, it has become possible to select the wavelength range, in which a toroidal grating spectrograph is near stigmatic, by adjusting the forces acting on the elastic grating substrate. The possibility of producing toroidal gratings by replication of an elastically deformed blank is pointed out.

Soft X-ray emission of continua from laser produced plasmas

Plasmas produced irradiating plane targets of low Z elements with moderate energy (3–10 J) lasers emit considerable amounts of free-free and free-bound continua in the soft X-ray region (10–100 Å) of the spectrum. With intensity calibrated grazing incidence spectrographs an intensity of the order of 108 W sr−1 Å−1 cm−2 emitted by the densest portion of the plasma inside the crater formed at the interaction was determined. This is in good agreement with theoretical predictions for a plasma of predetermined parameters. In case of high Z elements the continuum emitted has a very smooth wavelength dependence.

Photoionization and double excitation spectrum of Be2

Abstract The photoionization cross-section of Be 2+ has been measured in the range from threshold at 154 eV up to 420 eV. The value at threshold of [1.4 ± 0.2] × 10 −18 cm 2 is in good agreement with theoretical predictions. The method used was that of absorption using two laser produced plasmas. The doubly excited resonance 2s2p 1 p 0 at 281.25 ± 0.07 eV has been observed and its Fano profile measured.

Imaging extreme ultraviolet spectrometer employing a single toroidal diffraction grating: the initial evaluation.

A high-efficiency extreme ultraviolet (EUV) imaging spectrometer has been constructed and tested. The spectrometer employs a concave toroidal grating illuminated at normal incidence in a Rowland circle mounting and has only one reflecting surface. The toroidal grating has been fabricated by a new technique employing an elastically deformable submaster grating which is replicated in a spherical form and then mechanically distorted to produce the desired aspect ratio of the toroidal surface for stigmatic imaging over the selected wavelength range. The fixed toroidal grating used in the spectrometer is then replicated from this surface. Photographic tests and initial photoelectric tests with a 2-D pulse-counting detector system have verified the image quality of the toroidal grating at wavelengths near 600 A. The results of these initial tests are described in detail, and the basic designs of two instruments which could employ the imaging spectrometer for astrophysical investigations in space are briefly described, namely, a high-resolution EUV spectroheliometer for studies of the solar chromosphere, transition region, and corona and an EUV spectroscopic telescope for studies of nonsolar objects.

Stigmatic observations of laser-produced plasmas with a grazing-incidence spectrograph

A stigmatic spectrograph composed of a toroidal mirror and a concave grating both working at grazing incidence has been built and applied to the observation of laser-produced plasmas. Spatial resolution of 20-30 microm over a region of 1-mm extension and spectral resolution of approximately 1500 have been obtained with a considerable increase in speed of the spectrograph. By varying the relative positions of the optical elements, it is possible to change the wavelength of stigmatic condition in the range 10-300 A. Examples of observations of a beryllium plasma are shown.

Absorption spectra from 1s inner shell electron of ionized and neutral carbon

By using the technique of two laser-produced plasmas we have obtained the absorption spectra and the relative photoionization continua of ionized and neutral carbon in the soft X-ray region. In this spectral range (25-45 A), the absorption is due to the 1s inner shell electron. The CV absorption spectrum has been completely analyzed in a previous work and is reported here only as a reference spectrum for the CIV and CIII spectra. A preliminary analysis of the CIV is reported. Finally the absorption spectra and the relative photoionization continua of neutral carbon are presented in three different states, solid, vapour (clusters) and CI atoms.

Structure of the XUV emitting regions in a laser produced plasma

Abstract The XUV spectra emitted by plasma produced by focusing a ruby laser on plane targets of carbon and beryllium have been observed with a novel stigmatic spectrograph fitted with a toroidal mirror. The observations have been made both side-on on the expanding plasma and end-on looking inside the crater. In the latter case the intensity emitted is very large and the lines broadened indication electron densities approacing the critical density at 2.2×10 21 cm -3 . The relative intensity of resonance lines, free-bound and free-free continua and He-like and Li-like satellites has been determined. In particular the latter satellites appear to arise only from the region inside the crater.

An experiment for absorption spectroscopy in the XUV of light ions

The technique of using two laser-produced plasmas for absorption spectroscopy of moderately ionized species has been greatly improved. In particular a new signal detection system and a new data processing technique have been applied in the experiment with quite good results as regards both signal to noise ratio and spectral resolution. The absorption spectra of the 1s electron of the Be IV, III, II, I ions are shown together with the corresponding computed photoionization cross section spectra in the XUV as an example of the performances of the experiment.

An experiment for absorption spectroscopy of ionized species using two laser-produced plasmas

Abstract An experiment for obtaining the absorption spectra in the extreme ultraviolet of moderately ionized species is described. The technique relies on the use of two laser-produced plasmas, one acting as a background source and the second as the absorbing column. For the continuum producing source high Z materials are used as targets and a focusing technique at grazing incidence with a toroidal mirror is adopted for compensating the astigmatism of the grazing incidence spectrometer. The absorbing plasma is produced using a sphero-cylindrical lens. Absorption spectra of Be III, Be II, B IV and B III with high spectral resolution in the region 40–110 A are presented. Many new lines are observed in addition to the photoionization continua of Be III and B IV.