Let there be light! Fiat lux!

VUV direct-detection CCD

We used the McPherson 251MX soft x-ray spectrograph with aberration corrected gratings for improved performance AND collected deep UV spectra with a new EAGLE XO 4710 direct-detection CCD from Raptor Photonics in Northern Ireland. The 251MX is equipped with the new 120 g/mm diffraction grating that provides great wavelength coverage, 50~200 nanometers or more on a 25 mm wide sensor. The Raptor-Photonics EAGLE XO uses a 1024 x 1024 sensor with 13 micron square pixels (13.3 mm wide). It intercepts and sees about 80 nm wavelength range simultaneously. We did not need to use the deep cooling capability to see mercury emission at 184.9 nm or the complete deuterium atomic emission spectrum. We were pleasantly surprised that this combination of direct-detection CCD and grating allows for simplified setup too!

mcpherson monochromators and optic systems

#SXR #softXray #spectrograph #spectrometer #ultraviolet #CCD #camera #laser #optics #diffraction #UHV #OES #spectroscopy #hhg #fel #quantum #raptorphotonics #eagle #photemission #opticalemission #photoelectric #mcpherson #monochromator #calibration

Related posts:

  1. McPherson 251MX soft x-ray and XUV spectrograph
  2. Raptor Photonics Eagle XO data

Electron Penetration Depth

Anodes made from solid materials are used in the McPherson electron impact source for photo excitation and physics experiments in the SXR and XUV regime. They are excited by an e-beam with adjustable power. By calculating the penetration depth of exciting electrons, we can visualize the volume of emitting area! The electrons interact with the anode material and consequently decrease energy. Elastic and inelastic scatterings along the electron beam are described with the Bethe equation in power law [1]. The penetration of incident electrons is determined by ‘electron stopping power’ that decreases with increasing atomic number. Penetration depth of incident electrons is given by the Kanaya-Okayama formula [2].

solid anode elctron impact source

Ready for some fun? = R = 0.0276 * A * Eo^n / (z^0.89 * p)

R penetration depth, A atomic weight (g/mole), Eo electron beam energy, Z atomic number, p density (g/cm)^2, and n a constant, ~1.35 when the primary beam energy is < 5 keV, and be 1.67 when > 5 keV

#spectrometers #monochromators #optics #metrology #electromagneticspectrum #laser #xray #xuv #sxr #physics #quantum @nasagoddard @chandraxray

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  1. [1] H. Bethe, Handbook of Physics, Springer, Berlin Heidelberg New York, 1933
  2. [2] K. Kanaya, S. Okayama, J. Phys. D., J. Appl. Phys. 1972, 5, 43
  3. [3] Datasheet for 642 SXR light source
  4. [3] Datasheet for grazing incidence spectrometer for best spectral purity

Spectrophotometry in the XUV

We also build metrology equipment for spectroscopy applications in the deep ultraviolet wavelength region. These are generally spectrophotometers with optimized optical design and coatings, capable of operation with vacuum or inert gas purge. Spectrophotometry is a branch of electromagnetic spectroscopy concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. [1] This is how our customers use it too! Our most basic Vacuum Ultra-Violet Analytical Spectrophotometer (VUVAS, pronounce ‘voo-vaas’) can easily measure transmission in the 120~350 nanometer range. Also, reflection with goniometric control of sample and detector angles. We can measure reflection coatings at just about any angle and also dispersive optics, like diffraction gratings.

extreme ultraviolet spectrophotometer

The spectrophotometer systems can also be built up with different light source and monochromator sub components. One such alternate version with different monochromator/light source configuration works to 30 nanometers. [2] This system uses windowless light sources so a good working range is something like 30~160 nanometers. Pushing further to the soft x-ray and extreme ultraviolet is also done. These systems are built around grazing incidence monochromators that can reach short wavelengths, < 1 nm (!), using the electron impact x-ray source and glancing angle optics.

The McPherson XUV Spectrophotometer has been used to characterize optics, filters and detectors for projects like XMM-Newton, the Chandra X-ray Observatory and many more. Scientists like Neil deGrasse Tyson talk about “how much “more" we can see with Chandra than we can see with the human eye…and that different kinds of light are each a tool in the toolbox of astronomy." [Quote from 'Explore the X-ray Universe (4/19)'] In what portion of the electromagnetic spectrum we will be observing the universe next?

Related posts:

  1. [1] Wikipedia: Spectrophotometry
  2. [2] video on the McPhersonInc YouTube channel
  3. Vacuum Ultra-Violet Analytical Spectrophotometer (VUVAS, pronounce ‘voo-vaas’)

Radial Velocity Method, High-resolution Spectroscopy

I recently heard the expression ‘radial velocity method’ and learned of this techniques importance in detecting exoplanets. The radial-velocity method for detecting exoplanets uses Doppler Effect to sense a stars motion. Stars with companions, be they planets or other stars, move in response to the gravitational tug of their companion(s). Radial velocity measurements indicate stars that may have companion exoplanets and merit more detailed investigation.

Sensitivity is required for precise measurements. For example, the first Balmer line of hydrogen (Hydrogen−alpha, or Hα) is 656.3 nanometers. If we point our telescope at a star and measure Hydrogen−alpha with our well calibrated spectrograph, and detect it shifted 11 pm, at position 656.311 nanometers, we calculate radial velocity, vr, by Δλ / λ0 * c (1)

  • Δλ / λ0 = (656.311 – 656.3) / 656.3 = 0.0000167
  • vr / c = 0.0000167 * 299,724 km/s
  • velocity radial, vr = 5 km /sec

The sign of the result is positive, the observed star is moving away from us, a red shift at the time of measurement. A negative or blue shift indicates motion towards us.

A McPherson two meter focal length Echelle spectrometer served in a synoptic facility for solar observations. It made reliable, stable solar measurements at least twice daily for an eleven year solar cycle. Resolving powers were reported to be around 300,000 in 14th diffracted order(2). That solar cycle has ended. The instrument is still going. In first order – without use of Echelle gratings – the sensitivity is limited to about 2 pm in the ultraviolet(3). That correlates to about 2 km/s radial velocity and may be interesting for some researchers and science with readily available COTS instrumentation.

COTS spectrograph radial velocity method

Related posts:

  1. Doppler Effect and Readial Velocity Method by PSU
  2. NSO/NISP SOLIS Integrated Sunlight Spectrometer
  3. High resolution McPherson spectrograph

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Spectral Bandwidth & Resolution

Spectral bandwidth is important in spectroscopy. It is related to slit width and dispersion of the monochromator system. Both quantities rely on the groove density of the diffraction grating, the instrument focal length, and so on. Usually defined in nanometers, the spectral bandwidth is the measured width of a peak at half the maximum intensity (a.k.a. full-width-half-maximum or fwhm).

Spectral bandwidth affects accuracy and repeatability. For some analysis, insufficient ability to narrow bandwidth means that peaks and/or absorbance bands are not detectable. Small and sharp peaks will disappear, big and sharp peaks will become lower.

Here is a spectrum of a Deuterium lamp collected at less than 170 nanometers and with 1 nm and 0.1 nm bandwidth. Inset one can see 157~164 nm, the broader bandwidth shows two main peaks whereas finer bandwidth reveals the complex ‘many-line’ spectrum of hydrogen. High resolution and fine bandwidth may be needed for atomic spectroscopy, isotopes, and lanthanide applications.

The deuterium source is Model 634 and the monochromator Model 225 with adjustable slits.

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