摘要:

We describe a photoacoustic spectrometer optimized for the detection of nonradiative relaxation from molecular electronic states populated via two‐photon excitation. The excitation source is a nitrogen‐laser‐pumped dye laser; the analog data‐acquisition system automatically corrects the photoacoustic signal for pulse‐to‐pulse fluctuations in laser intensity; and the microcomputer interface is designed for universal communication to any microcomputer which supports an RS‐232C I/O port. As an example of the capabilities of this spectrometer, we present two‐photon photoacoustic spectra of CH3I and CD3I, covering small regions within the firsts‐Rydberg transitions, and show that the photoacoustic signal does indeed depend quadratically upon the incident laser intensity. These data represent the first direct detection of nonradiative decay from a molecular Rydberg state.

ISSN:0034-6748    

DOI:10.1063/1.1139584

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A simple method is proposed for the absolute calibration of photoacoustic cells using a condenser microphone as a calibrated sound source. This method allows the measurement of acoustical transfer function of resonant and nonresonant cells. By correcting experimental data with the transfer function a good agreement with theoretical previsions has been achieved. The technique was tested on an optically opaque thermally thick sample.

ISSN:0034-6748    

DOI:10.1063/1.1139585

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A high‐temperature microwave spectrometer has been constructed for the study of hot corrosive gases. The absorption cell consists of a 213‐cmX‐band copper waveguide which is heated along roughly 130 cm of its length. A thin nickel strip held taut by stainless‐steel posts at either end of the waveguide serves as the Stark septum. The sample chamber is coupled to the Stark cell midway along its length by a slip joint formed from two concentric tubes. This design ensures a long effective absorption path length and minimizes sample consumption. The hot cell is used in conjunction with an HP 8400C microwave spectrometer source which has been interfaced to a small computer for data acquisition, storage, and retrieval. The successful operation of the spectrometer has been demonstrated with KOH vapor at 580 °C. TheJ=2←1 transitions of a number of vibrational states of monomeric KOH have been observed in the region of 32.8 GHz. The observed signal‐to‐noise ratio of these transitions indicates that the sensitivity compares favorably to spectrometers which have been used to observe higherJtransitions in the submillimeter region. Radio‐frequency–microwave double resonance techniques have been demonstrated for the first time in a high‐temperature Stark cell by observing theJ=2←1l‐doublet transitions in the (011O) state of KOH.

ISSN:0034-6748    

DOI:10.1063/1.1139586

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

An ac technique is described for measuring low‐frequency resistance fluctuation spectra with improved sensitivity over dc methods achieved by avoiding preamplifier 1/fnoise. The technique, easily implemented with decade resistors and a lock‐in amplifier, allows the current noise of low‐resistance (r<10 k&OHgr;) specimens to be measured to frequencies below 1 mHz. Use of a center‐tapped, four‐probe specimen geometry allows discrimination between specimen and contact noise and eliminates noise due to bath temperature variations. The technique is demonstrated in use to determine the dependence of the 1/fnoise of Cr films on film area. Measurements with simultaneous direct and alternating currents provide means to study the noise of nonlinear devices and frequency‐dependent conductors.

ISSN:0034-6748    

DOI:10.1063/1.1139587

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A four‐probe technique for measurement of electrical resistance and photoconductivity on cryocrystals in a diamond‐anvil cell is described. The gasket and the method of fabrication of the electrical leads is discussed in detail. As an example, resistance and photoconductivity measurements on hydrogen iodide are presented.

ISSN:0034-6748    

DOI:10.1063/1.1139588

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Semiconducting Peltier devices are used as heat sources and sinks to apply sinusoidally varying temperature versus time profiles through square sheets of material. The thermal diffusivity may be determined by recording the temperature as a function of time at the interfaces of a stack of two or three sample sheets. The design of the experimental rig is described and its use is illustrated. The thermal diffusivity of a variety of poorly conducting materials such as ceramics and polymers, having a wide range of values of thermal diffusivity, may be measured using this approach by merely selecting the frequency and amplitude to allow for the thickness and material so as to be able to resolve the temperature phase differences and amplitudes at the junctions of the sheets. In this case, the technique is applied to the determination of thermal diffusivity of polyethylene by way of illustration of the method. Reproducibility and precision achieved throughout the temperature range 4–46 °C used was found to be no better than ±5%, but precision could be improved by interfacing the equipment with a computer. Good agreement was obtained with published values of thermal diffusivity.

ISSN:0034-6748    

DOI:10.1063/1.1139589

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A computer‐controlled apparatus for thermally stimulated polarization and depolarization current (TSPC/TSDC) measurements over the temperature range −196 to 250 °C is described. The instrument is designed to combine state‐of‐the‐art current sensitivity with computerized data acquisition and reduction. The apparatus is capable of detecting small (as low as 0.1 ppm) dipole concentrations (charge carriers) in solid dielectrics with the application of moderate electric fields (2–5 kV/cm).

ISSN:0034-6748    

DOI:10.1063/1.1139600

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A high‐pressure apparatus in the form of a modified Instron capillary rheometer capable of measuring differential thermal analysis (DTA) data and pressure–volume–temperature (P–V–T) relations was constructed in our laboratory. Temperatures from 20 to 370 °C and pressures from 1 to 6000 bar are achievable with a data‐acquisition and control system based on an APPLE II+ microcomputer. Measurements of pressure to an accuracy of 1%, temperature of 0.5%, and volume change of 0.1% have been obtained. Software was developed to operate the experiments at fixed heating or cooling rates as well as at a constant pressure or with isothermal pressure sweeps. Polymer samples were compressed into rods preceding the experiment by a vacuum molder to eliminate voids. Low‐density polyethylene (LDPE) was run as an example to demonstrate the performance of this pressure apparatus. The results revealed an excellent match between our experimental data and the published data.

ISSN:0034-6748    

DOI:10.1063/1.1139601

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

We describe a compact system providing the capability of striking a sample surface with a short pulse of reactive gas in an UHV environment and monitoring the evolution of ensuing volatile reaction products. A 300‐Hz rotating disk chopper synchronized with a pulsed valve produces molecular beam pulses of 12‐&mgr;s FWHM at the chopper position. Only one stage of differential pumping separates the pulsed valve chamber from the target chamber with the latter under UHV conditions. The source of the molecular beam, comprised of the chambers containing the pulsed valve and the rotating disk chopper, occupies less than one square meter of floor space. Beam characterization is accomplished by the use of two UHV compatible fast ion gauges located in the target chamber. Reaction products are monitored with microsecond resolution via a differentially pumped quadrupole mass spectrometer located at right angles to the beam.

ISSN:0034-6748    

DOI:10.1063/1.1139602

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A new compact detector for measuring the velocity distribution of He atoms scattered from an oriented single‐crystal surface by the time‐of‐flight (TOF) method is described. In this system the scattered atoms are tagged by exciting them to the 23Smetastable state by cross bombardment with a pulsed electron beam. After traversal of a given flight path (80.9 mm), the metastables are detected by a multichannel‐plate electron multiplier, and their TOF distribution is recorded. The incident supersonic He beam is generated by a nozzle source, resulting in a very narrow energy distribution of the incident beam. The angular and energy resolutions of the detector are analyzed and its advantages and limitations are described. Finally, a demonstration of the performance of the instrument in inelastic scattering from a Cu(100) surface is presented.

ISSN:0034-6748    

DOI:10.1063/1.1139603

出版商:AIP    

年代:1987

数据来源: AIP