NH and NH2 radical formation: photochemistry of NH3 versus N/N2+H/H2 radical addition reactions

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Infrared Fourier spectrometry (FTIR)

We know that spectroscopy studies the light absorption in the sample at different wavelengths. At first it was done by dispersing the light through a prism to different wavelengths and scanning the sample. Due to the fact that it takes long time to scan the whole diapason of wavelengths and requires good prisms and gratings, it is not used anymore.15 Right know Fourier spectrometers are used. They are based on electromagnetic wave interference. The negative aspects of the Fourier spectrometers are that they require difficult computations in calculating absorption spectra. At the present this is not an issue anymore since the science in computers is very advanced and the Fourier transformation takes a couple of seconds to calculate.
The main part of Fourier spectrometer is the interferometer. The most common are the Michelson interferometers (figure II-5a).16 It consists of light source, a beam splitter, a one fixed and one moving mirror system and a detector. Ideally 50% of the light is transmitted towards the fixed mirror and 50% is refracted towards the moving mirror. Light is reflected from the two mirrors back to the beam splitter and towards the sample and to the detector after that. The difference in optical path length between the two arms to the interferometer is known as the retardation or optical path difference Δx. An interferogram is obtained by varying the delay and recording the signal from the detector for various values of Δx. This results in a maximum at Δx = 0, when there is constructive interference at all wavelengths, followed by series of « wiggles »17 (as shown in figure II-5b). The interferogram has to be measured from zero path difference to a maximum length that depends on the resolution required. In practice the scan can be on either side of zero resulting in a double-sided interferogram from -Δxmax to Δxmax. For the interferogram to be converted to a spectrum a Fourier transformation is needed to be done.18

Advantages of Fourier transform spectroscopy

There are three main advantages for an FT spectroscopy compared to a scanning (dispersive) classical method:
Fellgett’s or multiplex advantage. One of the reasons FT spectrometers are much better than dispersive it that the information from all wavelengths is collected simultaneously. Obviously it allows shorter scan times. In practice multiple scans are often averaged where in the same time it would take for a single scan of a dispersive spectrometer. Jacquinot’s or throughput advantage. This results from the fact that in a dispersive instrument, the monochromator has entrance and exit slits which restrict the amount of light that passes through it. The interferometer throughput is determined only by the diameter of the collimated beam coming from the source. That means that the energy throughput in an interferometer can be higher than in a dispersive spectrometer. This combined with Fellgett’s advantage gives the ability to achieve the same signal-to-noise ratio as a dispersive instrument in a much shorter time.
Connes’ or the wavelength accuracy advantage. The wavelength scale is calibrated by a laser beam of known wavelength derived from a He-Ne (helium neon) laser that passes through the interferometer. This is much more stable and accurate than in dispersive instruments where the scale depends on the mechanical movement of diffraction gratings. The wavenumber of He- Ne laser is known very accurately and is very stable. As a result, the wavenumber calibration of interferometers is much more accurate and has much better long term stability than the calibration of dispersive instruments.

Calculation of absorbance spectrum of a sample

It was shown previously that using Lamber-Bouguer law it is possible to calculate absorbance for a specific wavelength. In ideal world it would be great, but in real world the spectrometers consists of parts (beam splitter, IR windows) that absorb some of the light in different wavelengths. So recording a reference spectrum is required.20 The reference measurement makes it possible to eliminate this instrument influence. The reference spectrum is a spectrum recorded without samples. Lamber-Bouguer law consists of two components referred as intensity, and these are obtained by recording reference and sample spectra. Later on using the same law the absorbance spectrum is calculated. All of this is shown in Figure II-6.

FTIR (Fourier Transform Infrared) spectrometer

In this equipment, all the infrared spectra were recorded using high resolution Bruker Vertex 80v43 Fourier transform spectrometer. For ice studies a mid-infrared (500 – 5000 cm-1) range is sufficient for vibrational identification of species. Most stretching and bending modes lie in this region. Optionally, if needed, this spectrometer is equipped with additional optical components to cover the whole spectral range from far infrared to ultraviolet (5 – 50000 cm-1).
As spectrometer is used under vacuum all the unwanted atmospheric interferents (water vapor, CO2) are eliminated from the spectrometer and there is no need for spectral manipulations like atmospheric compensation.44 Under these conditions measured IR spectrum will not contain residual H2O and CO2 features which could mask weak spectral bands of the sample. IR spectrometer compartment is separated from the vacuum chamber by a KBr window. The connected vacuum pump is oil-free in order to prevent oil vapor from entering the spectrometer optics interior.
This spectrometer works on principle of classical Michelson interferometer concept. The internal components are shown in figure II-19. Bruker Vertex 80v spectrometer consists of these components:
Light Source: The instrument is equipped with a MIR (mid infrared) source (H in figure II-19). The MIR light source is a globar (silicon carbide) that emits mid-infrared light.45 Optionally it can be equipped by other light sources (G in figure II-19) like mercury, tungsten or xenon sources. Detector: Same as a light source, a detector can be chosen depending on the study of the spectral region needed. This spectrometer is equipped with two detectors: MCT (HgCdTe) and InSb (B in figure II-19). For this study a MCT, liquid N2 cooled, detector was used because of its operating spectral range of 500 – 5000 cm-1. InSb detector is used for studies in near-infrared range (1850 – 10000 cm-1). Other detectors can be used for different spectral regions: Far Infrared – Bolometer (8 – 600 cm-1) or Visible and UV – Silicon diode (9000 – 25000 cm-1), GaP diode (18000 – 50000 cm-1).

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Interstellar ice analog formation

Due to the physical and chemical properties of the materials, experiments can be carried out in any state of the matter: gas, liquid or solid, depending on the experimental requirements. Atmospheric and interstellar studies used mainly solid and gas phase and eventually the interface solid-gas. The analysis of the investigated samples depends strongly on the temperature. At room temperature gas phase molecular spectra are complicated due to the high rotational and vibrational density states, which show numerous ro-vibrational transitions. This can be avoided by decreasing the temperature which strongly simplifies the structure of the molecular spectra. Solid phase molecular spectra show wide absorption bands which might overlap in different spectral regions. The transitions identified in solid phase are mainly vibrational transitions. Matrix isolation technique is usually used in order to reduce the environmental interactions and then to avoid overlapping band absorptions

Table of contents :

From a dream of a child to a scientific reality:
Chapter I: Introduction
I.1. Chemical evolution
I.2. Interstellar medium (ISM)
I.3. Formation of stars and planets
I.4. Interstellar chemistry
I.4.1. Gas phase reactions
I.4.2. Grain surface reactions
I.5. Thesis planning and description
Chapter II: Experimental section
II.1. Laboratory studies
II.2. Infrared spectrometry
II.2.1. Vibrations, absorbance, interferogram, fourier transformation
II.2.2. Molecular vibrations
II.2.3. Infrared absorption
II.2.4. Infrared Fourier spectrometry (FTIR)
II.2.5. Advantages of Fourier transform spectroscopy
II.2.6. Calculation of absorbance spectrum of a sample
II.3. Mass spectrometry
II.3.1 Mass selection
II.3.2. Molecular fragmentation
II.4. Experimental setup
II.4.1. Pumping system
II.4.2. Cryostat
II.4.3. Sample holder
II.4.4. Sample formation system (Ramps)
II.4.5. UV source unit
II.4.6. Microwave discharge source (MWD)
II.4.7. FTIR (Fourier Transform Infrared) spectrometer
II.4.8. QMS – Quadrupole mass spectrometer
II.5. Interstellar ice analog formation
II.5.1. Pure ices
II.5.2. Matrix isolation
II.6. Samples and materials: preparation and composition
II.6.1. Ramp volume and gas composition
II.6.2. Thickness of the ices
II.6.3. Chemicals used
Chapter III: Photochemistry of NH3-H2O ice
III.1. Earlier studies and motivation
III.2. NH3 photolysis in diluted phase
III.2.1. Sample formation and photolysis
III.3. Photolysis in concentrated phase of NH3 ice
III.4. Photolysis in concentrated phase of NH3-H2O ice
III.5. Formation of NH2OH from thermal processing of irradiated NH3-H2O ices
III.5.1. NH2OH formation pathways
III.6 From diluted (NH3-Ne-H2O) to concentrated phase (NH3-H2O ice)
III.7. NH and NH2 radical formation: photochemistry of NH3 versus N/N2+H/H2 radical addition reactions
III.7.1. Determining the optimal N2/H2 ratio for formation of nitrogen hydrates
III.7.2. Influence of water molecules on N/N2+H/H2 radical addition reactions
III.8. Conclusions
Chapter IV: Behavior of NH2OH in interstellar ice analogs
IV.1. Earlier studies and motivation
IV.2. Sample preparation: evaporation of NH2OH from the [NH2OH]3[H3PO4] salt
IV.3. NH2OH-H2O matrix isolation
IV.4. Formation of NH2OH-H2O ice
IV.4.1. Ice formation through a direct NH2OH-H2O deposition
IV.4.2. Ice formation from matrix isolated NH2OH-H2O
IV.5. Thermal processing of NH2OH-H2O interstellar ice analog
IV.6. Analysis of NH2OH thermal transformation in H2O ice
IV.7. Conclusions
Chapter V: Photochemistry of CH4-H2O ice
V.1. Earlier studies and motivation
V.2. Sample formation and identification of photoproducts
V.3. Photolysis of methane-water ice
V.3.1. Photolysis of methane-water ice: effect of water concentrations
V.4. Thermal processing of irradiated water-methane ices
V.4.1. Thermal processing of irradiated water-methane ices: alcohol identification
V.4.2. Alcohol identification: from solid to gas phase
V.5. Formation pathways of alcohols
V.6. Conclusions
Chapter VI: Formation of large alcohols under ISM conditions: Photolysis vs hydrogenation
VI.1. Earlier studies and motivation
VI.2. Sample formation
VI.3. Hydrogen addition reactions of unsaturated alcohols
VI.3.1. Hydrogenation of allyl alcohol (H2C=CHCH2OH):
VI.3.2. Hydrogenation of propargyl alcohol (HC≡CCH2OH)
VI.4. H addition reactions of unsaturated aldehydes
VI.4.1. Hydrogenation of propanal (CH3CH2CHO)
VI.4.2. Hydrogenation of propenal (H2C=CHCHO)
VI.4.3. Hydrogenation of propynal (HC≡CCHO)
VI.5. Transformation pathways
VI.6. Conclusions
Chapter VII: Photochemistry of CH4-NH3-H2O ice
VII.1. Earlier studies and motivation
VII.2. Sample formation and addition of water
VII.3. Identification of photolysis products in irradiated CH4-NH3-H2O ices
VII.4. Heating
VII.5. Transformation pathways
VII.6. Conclusions


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