Spectroscopic investigation of indium bromide for lighting purposes

In the modern world, the presence of light is taken for granted, but it comes at a cost. A large portion of the energy that is consumed by all of us today is used for lighting: 20% of the total electricity consumption. In a world where energy is becoming increasingly scarce and expensive, efficient lighting becomes more and more important. There are a few different lamp designs that are used to improve on the efficiency of the incandescent lamp: man’s first attempt at electric lighting. A new technology that is receiving a lot of attention currently is the light emitting diode (LED), and while this technology holds a great promise for the future, a major issue with this technology will be the purchase price per unit for the foreseeable future. Other commonly known technologies are the low pressure discharge lamp and the high pressure discharge lamp. The low pressure discharge lamp is known in the form of the fluorescent tube. This lamp works via a low pressure mercury discharge that produces UV-photons very efficiently that it then converts to visible light somewhat less efficiently. The high pressure discharge lamp is often seen in shop lighting. It works by the direct emission of visible light from a high pressure mercury discharge that has some (usually metal halides) additives in it. The high pressure discharge conditions make for a less efficient discharge. However, the light production efficiency of both these discharges is comparable (?? 100 lm/W).Ideally, a discharge lamp would combine a high discharge efficiency, like the low pressure discharge lamp has, with the direct production of visible light. To do this an intermediate pressure lamp is envisioned that produces light directly. The light producing species can not be the same as in the low or high pressure lamps. Indium Bromide is suggested as a model species to study. The spectrum of Indium Bromide will be studied to find optimal operating conditions as well as fundamental properties of the molecule. To do this, a laser induced fluorescence experiment is designed and built. A cylindrical tube with a diameter of 3 cm and a length of 30 cm is housed inside an oven that provides a homogenous temperature over the volume of the tube. Axally through the tube, a laser can be shone that provides excitation of the studied molecules. The laser used is either the 4th harmonic of a YAG laser or a high repetition rate (1 kHz) tunable dye laser which is pumped by the 3rd harmonic of a YAG pump laser. The high repetition frequency is necessary to prevent saturation effects, while still being able to measure meaningful fluorescence spectra. The fluorescence is detected in a perpendicular direction. The fluorescence photons are guided through a monochromator and then detected by either an iCCD camera for spectral resolution or a photomultiplier tube connected to a multichannel scaler for time resolution. In spectrally resolved LIF measurements both the excitation wave number and the detection wave number cover a certain wave number range. For the interpretation of the spectrally resolved LIF data, the so-called detex plots were introduced. A detex plot shows the fluorescence emission colorcoded in a graph with the detection wave length on the horizontal axis and the excitation wavelength on the vertical axis. These plots are very helpful for the interpretation of rovibronic spectra. The unprecedented detail visible in the detex plots enables a better unraveling of spectra than previously possible. This enabled the determination of various spectroscopic constants with greater accuracy. The detex plots helped produce lists of rotational transitions, however the beginning of these lists was generally missing. To still be able to assign the correct rotational quantum number to the observed transition, a method was developed based on the assumption that the unknown rotational constant must be similar to the rotational constant of a level nearby. This method can be useful in the determination of rotational constants for other species too.It can be seen that the decay time decreases gradually with increasing wave number. At higher wave numbers, and therefore higher rotational quantum numbers, the internuclear distance increases due to stretching of the chemical bonds in the molecule. In general, a higher internuclear distance is associated with a higher transition probability, but it also leads to a geometrically larger molecule. In other words, the collisional cross section for an InBr molecule in a highly excited rotational state may be larger than for a ground state InBr molecule. Two methods were used to determine the rotational temperature of a plasma:The intensity method, where the intensities of the rotational peaks is plotted and fitted with a modified Boltzmann distribution, and the fitting method, where the whole spectrum is fitted with a computer program based on the known spectroscopic constants. Both methods have been shown to be able to determine the temperature in a gas, where the accuracy of the temperature found by the fitting method seemed to be (predictably) slightly better. The fitting method yielded results with an accuracy of ?? 10-15%, whereas the achieved accuracy for the intensity method was of the order of ?? 25 - 30%. Using this method to determine the temperature in an inductive plasma proved to be impossible, due to the absence of InBr fluorescence in the region of interest. The experiments with the 266 nm excitation showed that collisions between InBr molecules contribute very significantly to the relaxation processes at temperatures typical for the operation of the envisioned lamp. This was confirmed by the measurements performed on the inductively coupled plasma, where molecular radiation was absent in the brightest part of the plasma. This leads to the conclusion that the molecular component in the radiation produced by an InBr discharge is (strongly) limited by the collisional relaxation processes. Indium itself is of course also a heavily radiating atom, so the use of Indium (perhaps in the form of InBr) as an additive to a chemical mix for a lamp is not categoricaly deemed as unwise. However the molecule InBr can be discarded as a candidate to be the primary radiator in an intermediate pressure lamp.