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obviously there is a possibility to speculate on quantum noise squeezing if factor @xmath143 is lesser than unity .
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calculations done for parameters of advanced ligo @xcite yield the value @xmath144 which means that despite the fact that limitations imposed on the antenna sensitivity by coating brownian noises are smaller then quantum noise imposed ones , their influence on the eigen mode amplitude are still comparable with the influence of latter .
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however the estimations for future antenna einstein telescope @xcite provide much more optimistic value @xmath145 .
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estimations for some experimental prototypes show that thermal noises are dominant in these devices .
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in particular for glasgow university prototype @xcite @xmath146 , for aei hannover prototype @xcite @xmath147 and for gingin facility @xcite @xmath148 .
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the parameters used for these estimations are listed in table [ tab : prototype ] .
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calculations for einstein telescope differ from another devices by used wavelength @xmath149 m and temperature @xmath150 k. .numerical parameters of advanced ligo ( aligo ) , einstein telescope ( et ) , glasgow prototype ( gp ) , aei hannover prototype and gingin high optical power test facility used for numerical estimations of thermal noises influence . [
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high - resolution infrared laboratory spectra are an essential element in numerous fields of science such as atmospheric chemistry @xcite and astronomy @xcite .
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these high - resolution spectra allow molecular emission and absorption features to be accurately identified and assigned ; for example the detection of ` water on the sun ' @xcite was made possible by the use of high - resolution hot water ( h@xmath9o ) line lists derived from experimental spectra .
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the h@xmath9o molecule is able to exist in sunspots ( typically 3,000 4,500 k ) , and observations showed that many previously unassigned lines were in fact due to the ubiquitous h@xmath9o molecule .
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molecules form in ` cool ' sources @xcite and progress in infrared astronomy has led to the identification of numerous molecules in cool stars @xcite , brown dwarfs @xcite and , most recently , in exoplanets @xcite .
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after h and he , c , n and o are the most abundant elements , so in cool objects one anticipates that methane ( ch@xmath10 ) , nh@xmath0 , and h@xmath9o will be abundant . although the spectra of the fundamental infrared bands of ` cold ' ( i.e. room temperature ) ch@xmath10 , nh@xmath0 , and h@xmath9o have been understood for many years , the situation for overtones , hot bands and combination bands is not so sanguine . for h@xmath9o
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it is only recently that high resolution experimental and theoretical line lists suitable for computing the spectral energy distributions ( seds ) of astronomical objects with temperatures in the 300 4000 k range have become available @xcite .
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for ` hot ' ch@xmath10 , there are only some rather sparse experimental observations and no satisfactory quantitative theoretical understanding . for hot nh@xmath0
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, there has been some recent theoretical progress @xcite and we report here on an extensive set of experimental observations in the thermal infrared .
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the nh@xmath0 molecule is one of the simplest polyatomic molecules with four atoms forming a trigonal pyramidal structure with c@xmath11 symmetry @xcite .
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nh@xmath0 has @xmath12 fundamental vibrational modes , of which two are doubly degenerate .
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there are therefore four fundamental vibrational frequencies @xmath13 at 3336.2 @xmath2 ( n - h stretch ) , @xmath14 at 932.5 @xmath2 ( umbrella mode ) , @xmath15 at 3443.6 @xmath2 ( n - h stretch ) and @xmath16 at 1626.1 @xmath2 ( bend ) , of which @xmath17 and @xmath5 are degenerate ( table [ tab1 ] ) .
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there is an additional complication in the spectroscopy of nh@xmath0 due to the low barrier to ` inversion ' .
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although nh@xmath0 is non - planar at equilibrium the three hydrogen atoms can flip through the plane containing the n atom and perpendicular to the c@xmath0 symmetry axis .
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lcrrl|lrl [ tab1 ] @xmath18 & @xmath19 & 3336.2 & ( 3337.2 ) & symmetric stretch & @xmath20 & 9.9443 @xmath2 + @xmath3 & @xmath19 & 932.5 & ( 968.3 ) & symmetric bend ( umbrella ) & @xmath21 & 6.196 @xmath2 + @xmath17 & @xmath22 & 3443.6 & ( 3443.9 ) & asymmetric stretch & @xmath23 & 1.0173 + @xmath5 & @xmath22 & 1626.1 & ( 1627.4 ) & asymmetric bend & @xmath24 & 107.8 @xmath1 + if the h atoms were labeled this would correspond to a right - handed form of the molecule converting into a left - handed form .
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hence , there are two forms ( frameworks ) of the nh@xmath0 molecule ( energy levels are conventionally labeled as @xmath25 and @xmath26 or + and - ) which can interconvert . indeed , the interconversion frequency at 24 ghz ( @xmath27 transition for @xmath28 , @xmath29 ) can be used by radio astronomers to monitor nh@xmath0 in dark interstellar clouds @xcite . in general , this inversion motion doubles vibration - rotation lines ( each of the two interconverting forms has slightly different energy levels ) and leads to the two slightly different band origins listed in table [ tab1 ] .
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nh@xmath0 is an oblate symmetric top ( @xmath30 @xmath2 , @xmath31 @xmath2 ) and the fundamental modes are either parallel ( @xmath18 and @xmath3 , @xmath32 ) or perpendicular ( @xmath17 and @xmath5 , @xmath33 ) depending on whether the infrared transition dipole moment is oscillating parallel or perpendicular to the c@xmath0 symmetry axis . the rotational quantum numbers @xmath34 and @xmath35 , describe the quantization of the total angular momentum ( excluding nuclear spin ) and its projection onto the symmetry axis , respectively .
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rotational selection rules ( @xmath36 , @xmath37 ) for a parallel transition lead to a simple @xmath38 , @xmath39 , @xmath40 structure ( @xmath41 ) with a small @xmath35 splitting . the selection rules for perpendicular transitions ( @xmath42 , @xmath37 ) lead to a more complex pattern with widely spaced @xmath35 structure , organized into @xmath35 sub - bands each with different @xmath43 values and @xmath38 , @xmath39 , @xmath40 structure @xcite . over the years
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, numerous studies have been undertaken to assign the complicated nh@xmath0 spectrum @xcite .
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many studies are of cold nh@xmath44 and are useful when studying planetary atmospheres in our solar system @xcite .
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some work has been done in determining empirical lower state energies @xcite as well as on the rotation - inversion spectra @xcite .
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recently there has been considerable progress in the variational calculation of vibration - rotation energy levels and transitions from adjusted ab initio potential energy surfaces @xcite .
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calculated line lists suitable for spectra of ammonia at 300 k @xcite and 1500 k @xcite are now available .
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cool objects are less luminous than hot stars and emit most of their radiation in the infrared .
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high resolution infrared spectra of these objects are best recorded with large 10 m class telescopes . in 1995 the first brown dwarf , gliese 229b was identified by the presence of methane in its spectrum @xcite .
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objects with masses less than 0.0075 solar masses are unable to fuse hydrogen in their cores @xcite and are called brown dwarfs .
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brown dwarf atmospheres are cool enough ( typically 700 2,000 k ) to sustain a rich molecular environment @xcite .
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since the discovery of gliese 229b , the l and t classes have been introduced to describe objects cooler than m - dwarfs @xcite .
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l - dwarfs display near infrared electronic transitions of metal hydrides such as feh @xcite and crh @xcite and have weak tio and vo transitions which are characteristic of m - dwarfs @xcite .
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in contrast , t - dwarfs like gliese 229b have prominent overtone transitions of hot h@xmath9o and ch@xmath10 @xcite . based on observations taken from the _ spitzer space telescope _
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, @xcite identified prominent h@xmath9o , ch@xmath10 and nh@xmath0 absorption bands between 6 14 @xmath4 m , including the nh@xmath0 @xmath3 band near 11 @xmath4 m .
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it is predicted that a new cooler classification , a y - dwarf ( @xmath45 700 k ) is required below the t - dwarf class @xcite with the prediction that strong nh@xmath0 absorptions will help to differentiate the y - dwarfs from t - dwarfs @xcite .
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numerous unassigned features are present in the observed seds of brown dwarfs . due to a lack of experimental data recorded in
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the temperature ranges appropriate for these objects ( or reliable theoretical predictions ) , the best option is to extrapolate room temperature ` cold ' line parameters such as the hitran 2008 database @xcite to the temperature of interest ( typically 1000 k for sub - stellar brown dwarfs ) .
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for this reason , these extrapolated spectra rarely match observations satisfactorily ( see figure [ fig1 ] ) due to the lack of hot bands and highly - excited rotational transitions which contribute significantly to the sed in these temperature ranges . [ fig1 ] along with brown dwarfs , it has only recently become possible to detect planets outside of the solar system ( exoplanets ) .
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51 pegasi b was the pioneering discovery by @xcite that paved the way for many more exoplanet detections , with over 500 discovered to date ( http://exoplanet.eu/ ) .
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the vast majority of these exoplanets are large gas giants close to their parent star with high temperature gaseous atmospheres typically referred to as ` hot jupiters ' .
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the new technique of ` transit spectroscopy ' @xcite has been used to probe exoplanet atmospheres @xcite and familiar molecules such as h@xmath9o , ch@xmath10 , co and co@xmath9 have all been shown to be present in examples like the well studied hd 209458b @xcite .
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the properties ( composition , temperature / pressure profiles , etc . ) of exoplanet atmospheres retrieved from the emergent flux or from transit spectroscopy depend on the molecular opacities used to simulate the observed spectra .
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spectroscopic studies of brown dwarfs and exoplanet atmospheres therefore need improved line parameters for hot nh@xmath0 and ch@xmath10 .
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currently the best line list for nh@xmath0 is the hitran 2008 database , which is incomplete and has insufficient hot band information for the astronomical applications considered here .
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moreover recent theoretical work has demonstrated that hitran 2008 has many errors in quantum number assignments of nh@xmath0 @xcite .
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nh@xmath0 has already been shown to contribute significantly to the sed of brown dwarfs at @xmath46620 k @xcite
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. improved theoretical predictions for the vibration - rotation lines will help to assign the complicated spectra of nh@xmath0 @xcite presented in this paper .
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we provide high temperature line lists for nh@xmath0 using a similar approach to that of @xcite ; in addition we derive empirical lower state energies for many of the lines .
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the line lists can be used directly to model the seds of brown dwarfs leading to a better understanding of the t-/y - dwarf boundary and perhaps allow the first identification of nh@xmath0 in an exoplanet atmosphere .
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laboratory emission spectra of hot nh@xmath0 were recorded using a system similar to that used by @xcite to study hot ch@xmath10 .
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a diagram of the experiment is shown in figure [ fig2 ] .
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an alumina ( al@xmath9o@xmath0 ) sample tube is sealed with windows and evacuated .
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a constant slow flow of nh@xmath0 gas is introduced to the system and a constant pressure is maintained using a needle valve which helps to minimize the build up of impurities and loss of sample within the system . surrounding the central 51 cm of the al@xmath9o@xmath0 tube ( 121 cm long ) is a controllable tube furnace capable of maintaining stable temperatures up to 1370@xmath1c .
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the radiation exiting the al@xmath9o@xmath0 tube is focussed into a fourier transform infrared ( ft - ir ) spectrometer using a lens and the distance between the window on the end of the al@xmath9o@xmath0 tube and the ft - ir spectrometer entrance aperture was 20 cm .
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this volume was purged with ` dry air ' ( h@xmath9o absent ) to minimize the presence of h@xmath9o absorption lines in the emission spectra .
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[ fig2 ] we recorded all spectra in two sections to improve the signal - to - noise ratio ( see figure [ fig3 ] ) and the experimental conditions for each region are listed in table [ tab2 ] .
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the two selected regions were a consequence of the available filters and constituent materials of the system .
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the first region covered the range 740 1690 @xmath2 and was recorded with thallium bromoiodide ( krs-5 ) windows , a potassium bromide ( kbr ) beamsplitter , a zinc selenide ( znse ) lens and a liquid nitrogen cooled mercury - cadmium telluride ( mct ) detector .
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the lower wavenumber cut off was limited due to the band gap of the mct detector and a small part of the nh@xmath0 @xmath3 umbrella mode could not be observed .
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[ fig3 ] lrr [ tab2 ] spectral region ( @xmath2 ) & 740 - 1690 & 1080 - 2200 + detector & mct & mct + beamsplitter & kbr & kbr + windows & krs-5 & caf@xmath9 + lens & znse & caf@xmath9 + scans & 240 & 80 + resolution ( @xmath2 ) & 0.01 & 0.01 + aperture ( mm ) & 3.15 & 2.5 + nh@xmath0 pressure ( torr ) & 5.0 & 1.0 + zerofilling factor & @xmath47 & @xmath47 + the second region covered the range 1080 2200 @xmath2 ( although no lines were observed above 2100 @xmath2 ) and was recorded with calcium fluoride ( caf@xmath9 ) windows , a kbr beamsplitter , a caf@xmath9 lens and liquid nitrogen cooled mct detector .
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