Infrared (IR) spectroscopy is another technique used to diagnose the gas-phase concentrations in a plasma and its advantage in comparison to the OES is that it can be used for any plasma condition (which is an issue with OES as explained in last section), for example dusty plasma or highly depleted plasma and, in general, even where the electron temperature varies. SiH4 has intense rotovibrational lines between 4.35 and 4.76 pm, and the v3 vibrational band of SiH4 consisting of P, Q and R branches between 2100 cm-1
and 2300 cm-1 has also been recorded . The infrared absorption can be measured in a simple way using a standard Fourier transform infrared (FTIR) spectrometer [58-60], though resolution in this case is rather low. An IR beam from a FTIR spectrometer, after a single pass through the reactor  or through a long gas line (typically 1m) after the reactor at the exhaust , can be detected at the other window and the resulting signal can be Fourier transformed to give the absorption spectrum, just as in case of a standard FTIR measurement. The measurement at the exhaust line of the deposition chamber is a nonintrusive, simple technique but is based on the assumption that the process pressure in the reactor is the same as in the exhaust (pumping) line and the silane and hydrogen partial pressures do not change between the reactor and the exhaust line. Nowadays, more accurate measurements are made using tunable lasers. In this case the gas is generally probed at the exhaust line of the chamber, before and after the plasma is on. The poor resolution does not allow the P and R branches of the v3 band to be used for any estimation, however, the integral of the Q branch shows a good correlation calibration with the silane density in the plasma. However, the availability of a proper laser source determines the range at which accurate spectra can be measured. For example, using a continuous-wave quantum cascade laser, spectra between 2241 and 2245 cm-1 belonging to the R (9) multiplet can be measured . The peak centered at 2243.827cm-1 is the one used as probe to study the gas condition. A good linear correlation between the intensity of this signal with the SiH4 flow rate (before ignition) has been observed. Comparing the peak intensity before and after the plasma is on, one can estimate the depletion (D) using the relation,
where AS“H4 and ASfH4 are the absorbances measured at a given wave number, for example 2243.827 cm-1, and п^щ4 and п^щ4 are the initial and depleted silane concentrations respectively and p!?iH4 and р!?ш4 are the respective partial pressures. Using the FTIR data, the growth rate can also be estimated using relationship,
where MSi and MSiH4 are the molar masses of silicon and silane respectively, mSiH4 is the mass flow rate of silane, Ar is the reactor surface (inner), and pSi is the mass density of silicon. This expression can be used to estimate any loss in the deposition rate due to formation of dust in the plasma. IR measurements have proved  that the dissociation rate is increased monotonously with increase in the plasma frequency and a fourfold increase in dissociation rate due to doubling of fractional depletion while passing from 13.56MHz to 70MHz proves why the deposition rate is high for VHF PECVD.