High-End Research Applications Using VERTEX Series Vacuum Spectrometers

What are the strong benefits of vacuum optics for demanding FT-IR experiments? The different vibrational modes of water vapour and carbon dioxide in the lab air have their absorption bands in the whole MIR (mid infrared) and FIR/THz (far infrared / terahertz) spectral range (see figure). Especially in the FIR region, the pure rotational modes of the atmospheric contaminations can even lead to total absorption of the IR light. The most common method of reducing the effects of atmospheric contaminations is to purge the optics bench as can be done for the VERTEX 80 and VERTEX 70 FT-IR spectrometers. However, even “dry” purge air always contains residual moisture and CO₂, causing significant atmospheric artifacts and effectively limiting sensitivity. In particular for demanding R&D measurements in mid and far infrared it can therefore be difficult or even impossible to obtain adequate results. Only a vacuum spectrometer can completely overcome these inherent limitations.

Bruker’s VERTEX 80v and VERTEX 70v vacuum FT-IR spectrometers provide highest flexibility and stability for advanced research applications. The entire vacuum optics design eliminates atmospheric disturbances in the resulting spectra and reduces artifacts caused by temperature fluctuations in the environment. Especially the VERTEX 80v vacuum spectrometer with its unique UltraScan^TM interferometer is the acknowledged gold standard for the most demanding applications, where highest sensitivity, broadest spectral range (in particular the FIR/THz region) or highest spectral or temporal resolution are required.

 Vacuum Advantages for FTIR Spectroscopy

• No temperature fluctuation influenced by environment
• No atmospheric absorbance masking weak spectral features or interfering with high spectral resolution features
• No problems caused by fluctuations of the dry air purge supply
• Highest stability and reproducibility
• Utmost sensitivity

VERTEX Vacuum Features

• Rapid evacuation by high speed, dry and noise-reduced vacuum pump
• Four position automatic beamsplitter (BMS) exchange option for the VERTEX 80v vacuum optics bench
• Automatic sample compartment shutter option for quick sample exchange and highest SNR
• Permanently mounted windows or adjustable telescope windows e.g. if particular experiments require a purged sample compartment

It is well-known, that VERTEX 80v research FT-IR spectrometers can access the THz spectral range down to 5 cm^-1 (approx. 0.15 THz) and hold the record for the broadest achievable spectral range from UV/Vis to THz using FT-IR technique. The vacuum spectrometers VERTEX 80v and VERTEX 70v can reach 10 cm^-1 using the room temperature FIR DTGS detector combined with the external Hg arc source and the appropriate beamsplitters. The superior instrument performance and sensitivity with this configuration are sufficient for most chemical or physics applications in FIR region. Furthermore, in some very demanding experiments requiring utmost sensitivity in the FIR/THz region to detect extremely weak spectral features, or for measurements requiring access down to a few wavenumbers, liquid Helium cooled bolometers can be applied additionally.

Outstanding FIR/THz spectral range specification of VERTEX vacuum spectrometers

• Superior sensitivity down to 50 cm^-1 using the RT DTGS detector
• Access down to 10 cm^-1 using additionally the external water-cooled Hg arc FIR source
• Numerous FIR BMSs for optimized performance and MIR-FIR wide range BMSs for easy operation available
• Record sensitivity in FIR/THz spectral range down to 5 cm-1 using VERTEX 80v and liquid He cooled bolometers

To reach the spectral limits, highest sensitivity or resolution in the FIR/THz region liquid Helium cooled bolometers are often required as detector. Since liquid He is very costly, in some regions even not available at all, the handling of this cryogen liquid demands skilled operators and significant set-up time, it is increasingly considered as bottle neck by many researchers. Alternative dry pulse tube cooled bolometers require long evacuation and cool down time (approx. 3-4 h), may create artifacts by potentially harmful vibrations and are still quite expensive.

Now these limitations are overcome by the new and unique verTera Terahertz extension for the VERTEX 80v vacuum spectrometer. With verTera functionality the famous VERTEX 80v becomes the world’s first and only combined FTIR/continuous wave THz spectrometer with amazing possibilities. A spectral range down to 3 cm^-1 (0.09 THz) can be covered without the need of any cryogenically cooled components.

Exciting possibilities with the verTera extension for VERTEX 80v

• Integrated state of the art cw THz technology
• Spectral range down to 3 cm^-1 (0.09 THz)
• Effective spectral resolution better than 0.0007 cm^-1 (better than 20 MHz)
• Same sample compartment and accessories for FT-IR and THz
• Highest THz performance by vacuum and unique THz algorithm
• Measurement of transmittance, reflectance and ATR
• Ready to measure by push of a button (no preparation time)
• FT-IR and THz mode controlled via powerful OPUS software

Why and when is the combination of ultra-high vacuum (UHV) and FTIR technique needed?

• The research topic requires very precisely defined conditions (such as pressure, concentration, temperature etc.) to study model systems and better understand real processes.
• The sample itself need to be cleaned, prepared, modified and/or kept under UHV conditions.
• Other combined analysis techniques (such as XPS, LEED, TDS etc.) in interdisciplinary research projects need preferentially UHV conditions.
• Not only UHV chambers, but also plasma chambers, high pressure chambers or other customized voluminous reaction chambers can be combined with FTIR technique.

 FT-IR spectroscopy can be added as a non-destructive and highly sensitive complementary analysis technique to a UHV facility or adapted to diverse customized chambers. Bruker’s VERTEX FT-IR vacuum series research spectrometers VERTEX 80v and VERTEX 70v with complete vacuum optics layout provide superior sensitivity, stability and reproducibility in comparison to purged spectrometers, because the entire beam path can be evacuated to avoid atmospheric and environmental disturbance. Especially the VERTEX 80v vacuum spectrometer is the gold standard for high-end FTIR applications and UHV adaptions allowing to measure weak bands down to 10^-5 au (absorbance units) and even beyond. Furthermore, the adaption of vacuum chambers with vacuum spectrometers is technically more efficient and reliable.

Bruker is highly experienced in this demanding application field and offers specialized UHV FT-IR solutions. We have successfully installed adaptions for different UHV systems manufactured by various UHV suppliers. We are able to offer flexible solutions to adapt UHV chambers of different dimensions and designs.

To fully understand your application challenges and meet your individual experimental requirements we provide you the UHV FT-IR Questionnaire to better express your need and enable smooth and efficient communication.

The step scan technique allows the monitoring of the temporal progress of very fast reproducible events. The interferometer mirror consecutively steps to the separate interferogram points, where the repeatable experiment is restarted again. All VERTEX series spectrometers can obtain superb time resolved data and superior stepping rates which are crucial for experimental feasibility. Thanks to its UltraScan^TM interferometer and the full-range of vacuum advantages the VERTEX 80v is widely accepted by the research community achieving unrivaled step-scan performance and most precise scanner control.

Why is vacuum important for step scan technique?

In VERTEX 80 and VERTEX 80v spectrometers, a mechanical positioning accuracy for the scanning mirror of better than 1 nm can be achieved. In such magnitude order already smallest temperature fluctuations of around 0.1 K will cause additional optical path variations of approx. 9 nm. Since in a purged instrument such fluctuations can never be excluded, only vacuum spectrometers can achieve effective accuracies in the low signal digit nm range. By combining the vacuum advantage and the unique UltraScan interferometer, the VERTEX 80v is therefore the only commercial system, achieving an effective positioning accuracy < 1 nm. Furthermore, it achieves the highest stepping rates of up to 50 steps/s, with a strong impact on complete measurement duration and general feasibility of experiments.

VERTEX 80v matchless step scan performance

• Highest stepping rate up to 50 steps per second
• Mechanical and effective positioning accuracy for the scanning mirror better than 1 nm
• Best performance for time resolved and amplitude/phase modulation spectroscopy
• Temporal resolution of 6 µsec using 24 Bit internal standard ADC
• Temporal resolution down to the low nsec range with transient recorder board, fast detector and fast preamplifier

There are different operation modes of step scan experiments. Time resolved step scan spectroscopy is used to follow very fast and repeatable reactions or processes e.g., the spectral emission and the pulse duration of a laser as shown in the figure. The stepwise data acquisition resulting in a matrix of interferogram points enable highest spectral resolution and highest temporal resolution at the same time. Amplitude modulated step scan can be used to highlight weak modulated signals from other surrounding signals, as it is applied for photoluminescence measurement in the mid IR region (details see application example MIR PL). Phase modulated step scan is e.g. required for example for depth profiling in photoacoustic spectroscopy.

Photoluminescence (PL) is an important analysis method in material/semiconductor sciences and optoelectronics. In the infrared spectral range the sensitivity of the FT-IR technique is significantly higher than for dispersive spectrometers. Bruker has decades of experience offering powerful PL solutions with the FT-IR research spectrometers.

For NIR PL the weak atmospheric absorptions of water vapor and CO₂ are not a big issue, so vacuum FT-IR spectrometers are not necessarily required. In the MIR region two additional challenges occur for PL experiments. First, atmospheric absorption is significantly stronger. Since PL measurements typically mean single channel spectroscopy there is no reference measurement to compensate the main part of atmospheric artifacts. Secondly, the LN2 cooled PL detector is sensitive to the MIR 300 K thermal background radiation, which will mask weak MIR PL signals. On account of this, amplitude modulated step scan must be used to get rid of the disruptive thermal background contribution.

To do so, modulated laser excitation is applied which in turn requires step-scan data acquisition. As a result also the PL signal will be modulated with the known modulation frequency of the excitation laser. Using state of the art dual channel electronics and lock-in techniques the modulated PL signal is then amplified while the constant and unwanted thermal background is suppressed and filtered out. Due to the above two challenges, vacuum spectrometers with dedicated vacuum PL module are highly recommended for MIR PL experiments. Since the entire beam path is under vacuum, atmospheric absorption can be completely eliminated. Furthermore, vacuum spectrometers and especially the VERTEX 80v has the best step scan performance for amplitude modulated experiments to suppress thermal background.

Ultra-thin layers on metal or dielectric substrates can be characterized in reflection mode using FT-IR technique. Due to the surface selection rules on metal substrates, s-polarized light cannot interact with the adsorbate molecules independent of incident angles, whereas p-polarized light reaches the maxima of absorbance at grazing incidence angle. Therefore, ultra-thin layers on metal substrates will be measured with Grazing Incidence Reflection (GIR) or Infrared Reflection Absorption Spectroscopy (IRRAS) using an incidence angle of around 80°.

Things become less obvious changing to non-metallic substrates, since both p- and s-polarized light can be absorbed by the thin layers. The intensity of the absorption bands varies depending on the incidence angle. Bands can even switch from negative to positive and vice versa by changing the incidence angle or polarization. Therefore, to fully characterize a thin layer on dielectric substrates, measurements using at least two incidence angles and with both polarizations must be carried out and also transmittance can be a valuable approach.

Ultra-thin layers show typically very weak absorption bands in IRRAS spectra, e.g. ranging down to 10^-3 au (absorbance unit) for a monomolecular layer with a thickness of few nanometers or even 10^-5 au on a dielectric substrate. For such weak absorbance bands highest sensitivity of the instrument is required. In the right side picture IRRAS spectra of a self-assembled monolayer on Au substrate are compared. The blue spectrum shows the result measured in a VERTEX 70 purge spectrometer. The residual water vapor absorbance masks the weak sample absorbance bands in the finger print region. After applying the automatic water vapor and CO₂ compensation function in OPUS software the red spectrum will be received.

Anyhow, only the result measured in a VERTEX 70v vacuum spectrometer (green spectrum) shows very smooth baseline, especially in the regions of atmospheric disturbance around 3700 cm^-1, 2300 cm^-1 and 1600 cm^-1. Furthermore, the spectrum measured in vacuum spectrometer is a pure experimental result without subsequent mathematical data manipulation as done for the red spectrum.

Electrochemical investigations are a very hot topic in basic and applied research. Recently, the world-wide trend of increasing energy consumption requires development of energy storage, e.g. high capacity and low-weight rechargeable batteries. Also in biochemistry or catalysis studies, electrochemistry is of great importance, to understand redox reactions and the behavior of catalysts. The combination of FT-IR spectroscopy with electrochemistry offers insight in the molecular change and the reaction process of the studied molecules in addition to the electrochemical response of the experiment.

 With Bruker’s reflection unit for electrochemical cells both reflection measurements to monitor changes at the surface of the working electrode and ATR measurements for investigation of electrolytes can be applied. In case of the reflection unit for vacuum spectrometers the entire IR beam path is under vacuum. However, the user has full access to the electrochemical cell from top of the unit, where the cell is adapted, with no need to open the sample compartment and to break the vacuum. It provides users the possibility to refresh the electrolyte solution or the electrodes for repeated or series of measurements keeping other experimental and measurement conditions constant. Furthermore, a higher sensitivity and signal to noise ratio can be achieved using a vacuum spectrometer, especially in the finger print region, because of the absence of atmospheric disturbances.

In many electrochemical investigations fast electrochemical response and reaction kinetics are in the center of interest. To follow the fast potential steps and collect a FT-IR spectrum at every applied potential value, just after equilibrium but before the potential is changed again for the next step, rapid scan is highly recommended and in many cases mandatory.

The result will be presented in an OPUS 3D view, not only showing the change along the wavenumber axis but also in dependence of the potential. In 3D-plot an example result from a redox reaction is shown. In this plot the change in absorbance of different vibrational bands during the measurement versus the time dependent potential is monitored. The reference has been measured once at the very beginning of the experiment. Therefore, in a purged FT-IR spectrometer the user will probably also see the change of the atmospheric absorbance during the whole electrochemical experiment interfering with the signal of interest. Although atmospheric compensation can be applied by software post processing, the result of a real vacuum measurement will always be superior to post processed data from a purged spectrometer. If a vacuum spectrometer is used, the user does not have to worry about atmospheric disturbance and fluctuation of purge conditions anymore. No subsequent data manipulation will be necessary ensuring highest sensitivity and stability for your research work.

The VERTEX series vacuum spectrometers VERTEX 80v and VERTEX 70v show their superiority in diverse other application fields:

• Low temperature spectroscopy
• Semiconductor/material science
• Matrix isolation spectroscopy
• Solid state physics
• Meta materials development
• Detector characterization
• Source characterization