This PhD thesis deals with phenomena which are closely related to the unique thermal structure of the polar summer mesosphere, namely Polar Mesosphere Summer Echoes (PMSE). PMSE are strong radar echoes commonly observed by VHF MST radars from thin layers in the 80-90 km altitude interval at high latitudes during summer. They follow a seasonal pattern of abrupt appearance in late May and a gradual disappearance in mid-August. This period corresponds roughly to the time between the completion of the summer time cooling of the polar mesopause to the time of reversal of the mesospheric circulation to autumn condition. In this connection, PMSE are associated with the extremely low temperatures, i.e. below 140 K, which are unique to the polar summer mesopause. Traditional theories of radar (partial) reflection and scattering have been unable to explain the PMSE and the exact mechanism for their occurrence remains unclear despite the steadily increasing interest in them over the past 20 years. Currently accepted theories regarding the mechanism giving rise to PMSE agree that one of the conditions needed for enhanced radar echoes is the presence of low-mobility charge carries such as large cluster ions and ice aerosols which capture the ambient electrons. It has been established that the PMSE are in some way associated with noctilucent clouds (NLC), layers of ice crystals, which constitute the highest observed clouds in the earth’s atmosphere. PMSE occurrence and dynamics are also found to be closely connected with the planetary and gravity waves. Observations of PMSE presented in this thesis have been carried out by the Esrange MST radar (ESRAD) located at Esrange (67°56’N, 21°04’E) just outside Kiruna in northernmost Sweden. The radar operates at 52 MHz with 72 kW peak power and a maximum duty cycle of 5%. The antenna consists of 12x12 array of 5-element Yagis with a 0.7l spacing. During the PMSE measurements the radar used a 16-bit complementary code having a baud length of 1mS. This corresponds to height resolution of 150 m. The sampling frequency was set at 1450 Hz. The covered height range was 80-90 km. The presence of PMSE was determined on the basis of the radar SNR (signal-to-noise ratio). The PMSE measurements have been made during May-August each year since 1997. PMSE seasonal and diurnal occurrence rates as well as dynamics have been studied in connection with tidal winds, planetary waves, temperature and water vapor content in the mesosphere (Papers I, IV and VI). Simultaneous and common-volume observations of PMSE and noctilucent clouds have been performed by radar, lidar and CCD camera (Paper V). Correlation between variations in PMSE and variations in extra ionization added by precipitating energetic electrons or high-energy particles from the Sun has been examined (Papers II and III). Possible influence of transport effects due to the electric field on PMSE appearance has been studied during a solar proton event (Paper III).

A meteor observed with the naked eye is colloquially called a shooting star. The streak of light is generated by an extra-terrestrial particle, a meteoroid, entering the Earth’s atmosphere. The term meteor includes both luminosity detectable by optical means and ionization detectable by radar. The radar targets of meteor head echoes have the same motion as the meteoroids on their atmospheric flight and are relatively independent of aspect angle. They appear to be compact regions of plasma created at around 100 km altitude and have no appreciable duration. This thesis reviews the meteor head echo observations carried out with the tristatic 930 MHz EISCAT UHF radar system during four 24h runs between 2002 and 2005, and a 6h run in 2003 with the monostatic 224 MHz EISCAT VHF radar. It contains the first strong observational evidence of a submillimeter-sized meteoroid breaking apart into two distinct fragments. This discovery promises to be useful in the further understanding of the interaction processes of meteoroids with the Earth’s atmosphere and thus also the properties of interplanetary/interstellar dust. The tristatic capability of the EISCAT UHF system makes it a unique tool for investigating the physical properties of meteoroids and the meteor head echo scattering process. The thesis presents a method for determining the position of a compact radar target in the common volume of the antenna beams and demonstrates its applicability for meteor studies. The inferred positions of the meteor targets are used to estimate their velocities, decelerations, directions of arrival and radar cross sections (RCS) with unprecedented accuracy. The head echoes are detected at virtually all possible aspect angles all the way out to 130° from the meteoroid trajectory, limited by the antenna pointing directions. The RCS of individual meteors simultaneously observed with the three receivers are equal within the accuracy of the measurements with a very slight trend suggesting that the RCS decreases with increasing aspect angle. A statistical evaluation of the measurement technique shows that the determined Doppler velocity agrees with the target range rate. This demonstrates that no contribution from slipping plasma is detected and that the Doppler velocities are unbiased within the measurement accuracy. The velocities of the detected meteoroids are in the range of 19-70 km/s, but with very few detections at velocities below 30 km/s. The thesis compares observations with a numerical single-body ablation model, which simulates the physical processes during meteoroid flight through the atmosphere. The estimated meteoroid masses are in the range of 10-9 - 10-5.5 kg. / Meteorer är ljusfenomen på natthimlen som i vardagligt tal kallas fallande stjärnor. Ljusstrimmorna alstras av meteoroider, små partiklar på banor genom solsystemet, som kolliderar med jordens atmosfär. Förutom ljus genererar meteoroider regioner av joniserat plasma, som är detekterbara med radar. Meteoriska huvudekon tycks komma från kompakta radarmål på ungefär 100 km höjd och rör sig genom atmosfären med de infallande meteoroidernas hastighet. Huvudekons signalstyrka förefaller oberoende av vinkeln mellan radarmålens rörelseriktning och riktningen från vilken radiovågorna infaller och sprids. Avhandlingen sammanfattar huvudekoobservationer från fyra 24-timmarsmätningar mellan 2002 och 2005 med det trestatiska 930 MHz EISCAT UHF-radarsystemet och en 6-timmarsmätning under 2003 med den monostatiska 224 MHz EISCAT VHF-radarn. Avhandlingen innehåller den första observationella bekräftelsen på att en meteoroid av sub-millimeterstorlek faller sönder i två distinkta fragment i atmosfären. Upptäckten är betydelsefull för studier av meteoroiders växelverkansprocesser med atmosfären och interplanetärt/interstellärt stofts materialegenskaper. EISCAT UHF-systemet består av tre vitt åtskilda mottagarstationer, vilket gör det till ett unikt mätinstrument för studier av meteoroiders egenskaper och hur radiovågor sprids från de radarmål som ger upphov till huvudekon. Avhandlingen presenterar en metod med vilken ett radarmåls position kan bestämmas om det detekteras simultant med de tre mottagarna. Metoden används till att med hög noggrannhet beräkna meteorers radartvärsnitt samt meteoroiders hastighet och atmosfärsinbromsning. De detekterade huvudekona genereras av meteoroider med i princip alla av mätgeometrin tillåtna rörelseriktningar i förhållande till radarstrålen, ända ut till 130° från radiovågornas spridningsriktning. Enskilda meteorers radartvärsnitt är likvärdiga inom mätnoggrannheten i de tre mottagarstationernas dataserier, men en svag trend antyder att radartvärsnittet minskar med ökande vinkel mellan meteoroidernas rörelseriktning och spridningsriktningen. En statistisk utvärdering av mättekniken visar att den uppmätta dopplerhastigheten stämmer överens med radarmålens flygtidshastighet. Detta innebär att dopplerhastigheterna är väntevärdesriktiga och opåverkade av bidrag från det spår av plasma som meteoroiderna lämnar efter sig. De uppmätta hastigheterna är 19-70 km/s, men bara ett fåtal detekterade meteoroider är långsammare än 30 km/s. Meteoroidmassorna är uppskattade till 10-9 – 10-5.5 kg genom jämförelser av observationerna med simuleringar av meteoroiders färd genom atmosfären i en numerisk ablationsmodell.

This thesis has two major objectives. The first objective is to investigate the seasonal and diurnal variations in occurrence of HF coherent echoes. We assess F-region echo occurrence rates for the PolarDARN HF radars at Inuvik (INV) and Rankin Inlet (RKN) and the auroral zone SuperDARN radars at Saskatoon (SAS) and Prince George (PGR) for the period of 2007-2010. We show that the INV and RKN PolarDARN radars show comparable rates of echo occurrence all the time and they detect 1.5-2.5 times more echoes through ½-hop propagation mode (MLATs=80°-85°) than the SAS and PGR SuperDARN radars through 1½-hope propagation mode (MLATs=75°-80°). For all four radars, the winter occurrence rates are about ~2 times higher than the summer rates. For observations in the dusk, midnight and dawn sectors, equinoctial maxima are evident. The pattern of echo occurrence in terms of MLT/season is about the same for all radars with clear maxima near noon during winters and summers and enhanced (as compared to other time of the day) occurrence rates during equinoctial dusk and dawn hours. Additionally, to investigate the effect of solar cycle on occurrence of F-region echoes, we consider the near noon and near midnight echo occurrence rates for the Saskatoon radar over the period of 1994-2010. We show that there is a strong, by a factor of ~10, increase in SAS night-side echo occurrence towards solar maximum. The effect does not exist for the dayside echoes; moreover, a decrease in number of echoes, by a factor of ~2, was discovered for the declining phase of the solar cycle. The second objective is to evaluate the electron density and the electric field as factors controlling the occurrence of F-region echoes. We use observations of these two ionospheric parameters measured by CADI ionosonde and RKN observations of echo occurrence rates over Resolute Bay (MLAT=83°). We show that there is a correlation in changes of echo occurrence and electron density changes for 3 years of radar-ionosonde joint operation (2008-2010). The comparison of radar-ionosonde data shows that the enhanced echo occurrence at near noon hours during summer months correlate with the enhanced electric field during these periods.

A ênfase deste trabalho foi a aplicação da técnica de RMN em campo zero para a obtenção do acoplamento quadrupolar e dos espectros de RMN em banda larga dos núcleos de 27Al (amostra GdAl2), 139La e 55Mn (amostras de Manganita), à temperatura de 4,2 K. O objetivo do trabalho foi obter resultados confiáveis tanto em relação aos espectros quanto aos valores dos acoplamentos quadrupolares das amostras de GdAl2 e de Manganitas, e utilizar estes resultados para auxiliar a entender e a determinar algumas das propriedades físicas destes materiais (no caso do GdAl2 o comportamento tipo Spin-Glass e o efeito Magnetocalórico, no caso das Manganitas a Magnetoresistência Colossal). Para se determinar o valor do acoplamento quadrupolar utilizou-se a técnica de medida das oscilações quadrupolares. Os espectros de RMN, em banda larga, foram obtidos utilizando-se a técnica de eco de Hahn variando-se a frequência de excitação em uma faixa de 5 a 500 MHz. A maior sensibilidade do equipamento montado em nosso laboratório permitiu que se observassem os múltiplos ecos devidos à interação quadrupolar. Consequentemente, estes ecos permitiram a obtenção de espectros de RMNz de 27Al com alta resolução e a medida do valor do acoplamento quadrupolar diretamente do espectro. Também foi mostrado que os múltiplos ecos do 27Al são originados em regiões diferentes da amostra. Os resultados dos experimentos de oscilação quadrupolar em amostras de Manganitas permitiram que se observassem vários valores de acoplamento quadrupolar, que podem ser devido à desordem da estrutura cristalina dos octaedros de MnO6 ou à variação angular do ângulo entre o campo magnético hiperfino e a o eixo de fácil magnetização. Também foi mostrado que os valores de acoplamento quadrupolar mudam em função do elemento dopante utilizado. A aplicação do campo externo diminui a distribuição de acoplamento quadrupolar, mostrando que a aplicação do campo magnético externo causa mudanças ou na rede cristalina e/ou na orientação do campo magnético hiperfino e, consequentemente, sobre as propriedades físicas das Manganitas. A comparação entre os espectros de 139La e 55Mn das diferentes amostras de Manganita mostrou a coexistência de fases magnéticas. Estes resultados mostram que a técnica de RMNz é uma excelente ferramenta para se determinar o estado magnético fundamental dos materiais magnéticos e as mudanças na estrutura cristalina através da observação do acoplamento quadrupolar. / The emphasis of this work was the application of zero-field NMR technique to obtain the quadrupole coupling and NMR broadband spectra of the nuclei 27Al (GdAl2 sample), 139La and 55Mn (Manganites samples), at 4.2 K. The objective was to obtain reliable results for both spectra and quadrupole coupling values of samples GdAl2 and Manganites, and use these results to help understand and determine some of the physical properties of these materials (for GdAl2 the Spin-Glass type behavior and the Magnetocaloric effect and for Manganites the Colossal Magnetoresistance effect). To determine the value of the quadrupole coupling it was used the quadrupole oscillations technique. The NMR broadband spectra were obtained using the Hahn echo technique, in the frequency excitation range 5 to 500 MHz. The greater sensitivity of the equipment allowed observing multiples echoes due to quadrupole interaction, these echoes allowed to obtain spectra of zero field RMN of 27Al with high resolution and measure the value of the quadrupole coupling directly from the spectrum. It was also shown that the 27Al multiple echoes are generated in different regions of the sample. The quadrupole oscillation experiments results on Manganite samples have obtaned several values of quadrupole coupling, indicating that the Manganites Ferromagnetic Metallic Phase present high degree of disorder or, in the crystalline structures or in the hyperfine magnetic field direction. It was shown that the change in the value of the quadrupole coupling depends on the dopant used in the Manganites composition. The results of the quadrupole oscillations with application of an external magnetic field showed that the degree of disorder decreases due to the external magnetic field, showing the influence of the external magnetic field on the crystal network and/or in the hyperfine magnetic field direction, and consequently of some Manganites physical properties. The comparison between the spectra of 139La and 55Mn of the different Manganite samples shows the coexistence of different magnetic phases. These results showing that the technique of zero-field NMR is excellent to determine the fundamental magnetic state of the magnetic materials and observe changes in the crystal structure by the observation of the quadrupole coupling.

Neste trabalho desenvolvemos metodologias para o estudo de materiais magnéticos usando Ressonância Magnética Nuclear em campo magnético externo nulo (RMNz), via a análise e manipulação das transições entre os estados de energia nucleares. Na primeira etapa, revisamos e complementamos os modelos teóricos, encontrados na literatura, que são utilizados para prever a formação dos múltiplos ecos. Concluímos deste estudo, que o aparecimento destes múltiplos ecos está associado com as transições entre os níveis de energia nuclear que são chamadas de transições de múltiplo quantum (MQ). Adicionalmente, concluímos que os instantes nos quais surgem os ecos de múltiplo quantum e os tipos de coerências que contribuem para cada eco estão diretamente relacionados com as distribuições dos acoplamentos Zeeman e/ou quadrupolar ao longo da amostra. Na segunda etapa deste projeto estudamos a seleção de coerências mediante a aplicação de ciclagem das fases dos pulsos de radiofrequências. Além de permitir o estudo individual de cada eco, a seleção de coerências permitiu a realização de medidas com tempos ao eco muito mais curtos e, consequentemente, a medida de sinais com maiores relações sinal/ruído. O que permitiu o estudo seletivo de cada eco, em detrimento dos outros, e assim pudemos inferir que coerências contribuem para formação do eco selecionado. Os compostos usados para o estudo experimental foram o GdAl2 e GdCo2. Os sinais de RMN dos núcleos 155,157Gd, 27Al e 59Co apresentaram ecos de múltiplo quantum. Posteriormente realizamos a seleção de coerências para estes núcleos. / This study we developed methodologies for the study of magnetic materials using nuclear magnetic resonance without the application of external magnetic field (RMNz), by manipulating transitions between the nuclear energy states. In the first step, we revised and complemented the theoretical models found in the literature, which were proposed to predict the formation of multiple echoes. We concluded from this study that the appearance of multiple echoes is associated with transitions between the levels of nuclear transitions that are called multiple-quantum (MQ). Moreover, we understood that the instants in which the multiple quantum echoes appear and the types of coherences that contribute to each echo are directly associated with the Zeeman and electric quadrupolar coupling heterogeneities along the sample. In the second step of this project, we studied the selection of coherences by phase cycling the radiofrequency pulses. This allowed the individual study of each echo, enabling us to infer which coherences contribute to the formation of the selected echo. Besides the individual study of each echo, the coherence selection allowed measuring them with the smallest echo time and, consequently, observing signals with much higher signal-to-noise ratios. The compounds used for the experimental study were the GdAl2 and GdCo2. The NMR signals of nuclei 155,157Gd, 27Al and 59Co showed multiple quantum echoes. Later we performed the selection of coherences for these nuclei.

Cardiac magnetic resonance (MR) imaging can non-invasively assess heart function. Displacement encoding with stimulated echoes (DENSE) is an advanced cardiac MR imaging technique that measures tissue displacement and can be used to quantify cardiac mechanics (e.g. strain and torsion). When combined with clinical risk factors, cardiac mechanics have been shown to be better predictors of mortality than traditional measures of heart function. End-expiratory breath-holds are typically used to minimize respiratory motion artifacts. Unfortunately, requiring subjects to breath-hold introduces limitations with the duration of image acquisition and quality of data acquired, especially in patients with limited ability to hold their breath. Thus, DENSE acquisitions often require respiratory navigator gating, which works by measuring the diaphragm during normal breathing and only acquiring data when the diaphragm is within a pre-defined acceptance window. Unfortunately, navigator gating results in long scan durations due to inconsistent breathing patterns. Also, the navigator echo can be used in different ways to accept or reject image data, which creates several navigator configuration options. Each respiratory navigator configuration has distinct advantages and disadvantages that directly affect scan duration and image quality, which can affect derived cardiac mechanics. Scan duration and image quality need to be optimized to improve the clinical utility of DENSE. Thus, the goal of this project was to optimize those parameters. To accomplish this goal, we set out to complete 3 aims: 1) understand how respiratory gating affects the reproducibility of measures of cardiac mechanics, 2) determine the optimal respiratory navigator configuration, and 3) reduce scan duration by developing and using an interactive videogame to optimize navigator efficiency. Aim 1 of this project demonstrated that the variability in torsion, but not strain, could be significantly reduced through the use of a respiratory navigator compared to traditional breath-holds. Aim 2 demonstrated that, among the configuration options, the dual-navigator configuration resulted in the best image quality compared to the reference standard (traditional breath-holds), but also resulted in the longest scan duration. In Aim 3, we developed an interactive breathing-controlled videogame and demonstrated that its use during cardiac MR can significantly reduce scan duration compared to traditional free-breathing and also led to a small improvement in signal-to-noise ratio of the acquired images. In summary, respiratory navigator gating with DENSE 1) reduces the variability in measured LV torsion, 2) results in the best image quality with the dual-navigator configuration, and 3) results in significantly shorter scan durations through the use of an interactive videogame. Selecting the optimal navigator configuration and using an interactive videogame can improve the clinical utility of DENSE.

Classical theory cannot provide a satisfying scenario for a unitary thermodynamic evolution of black holes. To preserve information one requires quantum mechanical effects on scales reaching beyond the traditional horizon radius. Therefore, common to many of the theories attempting to resolve the paradox is the existence of exotic horizon structures. The recent advent of gravitational wave astronomy provides a possible means for detecting the existence of such structures through gravitational wave emission in the ringdown phase of binary black hole mergers. Such emission is described by quasinormal modes (QNMs) in which the gravitational waves originates outside the black hole, in the vicinity of the photon spheres. Requiring reflective properties of the horizon structure results in the existence of gravitational echoes that may be detected by facilities such as LIGO etc.. This thesis studies geodesic motion of such echoes in the equatorial plane of a rotating black hole. Depending on the extent of the horizon structure, and the particular mode of emission, one can expect different timescales for the echoes. For a horizon structure extending <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5CLambda%20r%20=" /> <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?10%5E%7B-12%7D" /> <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?M" /> outside the traditional horizon of a  <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?M%20=%2022.6%20M_%7B%5Codot%7D" />, <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?a%20=%200.74%20M" /> black hole one would ideally find echoes appearing as integer multiples of <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5CDelta%20t_%7Becho%7D" /><img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?=%200.0204%20s" /> after the primary signal. The time delay is expected to increase by at least an order of magnitude if one lets <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5CDelta%20r%20%5Csim%2010%5E%7B-80%7D%20M" />. The expected echo timescales for gravitational waves emitted from any point around the black hole, in arbitrary modes, is an interesting further study.

This Master thesis examines a phenomenon that occurs in the upper polar atmosphere, namely, Polar Mesospheric Winter Echoes, or PMWEs. PMWEs are radar echoes observed by Very High Frequency Mesosphere Stratosphere Troposphere (VHF MST) radar, from altitudes of 60 to 76 km at 7 to 15 UT, in the winter months from the middle of September to the beginning of May. The aforementioned specifications are the partial results of this thesis.PMWEs are generally understood to be caused by turbulence; however, radar data indicate some rare cases where PMWEs can be created with velocities exceeding the speed of sound, which is not possible with current turbulence theory. Kirkwood et al., 2006 and Belova, Kirkwood, and Sergienko, 2013 hypothesised that infrasound could generate the necessary conditions for PMWEs with velocities equal to or exceeding the speed of sound. Observations of PMWEs presented in this thesis have been carried out by the MST radar ESRAD, located at Esrange (67 56’N, 21 04’E) near Kiruna in northern Sweden. The radar operates at 52 MHz and has been performing continuous radar observations since Dec 1996. Observations of the infrasounds presented in this thesis were carried out by a microbarometer located close to Rymdcampus in Kiruna. Access to the data is restricted, but through university administration, this master’s thesis has been granted permission to use the data for the study of PMWEs. The instrument performed continuous infrasound measurements from the 24th of May 2016 and forward. This thesis will perform a full analysis of all radar data from the 17th of Dec 1996 until the 31st of Jan 2021 to assess the altitude interval, diurnal interval and yearly interval. The data given in the first section are a result of this work. Additionally, space weather parameters relation with the occurrence of PMWEs is analysed. Space weather parameters are very important for the chemistry and conditions present in the mesosphere. It is shown thatt here is a strong relation between solar wind and PMWE occurrence, decent relation with Kp index and no to weak relation with solar particle event (SPE). Correlating space weather and PMWEs in greater detail could be the subject of other studies. Last but most interesting, microbarometer data will be analysed for days where high-speed PMWEs are detected. Because of the rarity of these high-speed PMWEs, only seven total cases were found from 24 May 2016 onward, making the analysis of the infrasound measurements very limited, and no connection was found. However, it was found that days with high-speed PMWEs had an abnormally low amount of infrasound detections, further making analysis difficult but raising questions of why. This could mean that infrasound signals might be hindered from reaching the ground on days where we have high-speed PMWEs. No further conclusions can be made, as this indicates relation but not causation. Airborne infrasound instruments could be used to detect weaker infrasound signals due to being unaffected by wind disturbance at the ground.

No description available.

Nous avons mis en place un dispositif permettant l’acquisition d'échos de photons stimulés infrarouges à l’échelle femtoseconde. Le but est d'examiner la dynamique vibrationnelle aux temps courts de métaux carbonyles (W(CO)₆ and Fe(CO)₅) piégés en matrice cryogénique (4-50 K). Cet environnement solide, issu de la condensation d'un mélange gazeux contenant une impureté et un gaz inerte (N₂, CH₄, Ar, etc.), est propice à l’étude de systèmes dans leur état fondamental. L’excitation d’une vibration moléculaire s’atténue toujours temporellement, ce qui correspond dans le domaine spectral à un élargissement des raies d’absorption. L’étude de la dynamique vibrationnelle vise à examiner les causes physiques à l’origine de cet élargissement spectral. Typiquement, elles sont de trois sortes : phénomènes intramoléculaires, interactions entre molécules piégées et interactions entre la molécule piégée et l’environnement. Les échos de photons permettent de distinguer les contributions homogènes et inhomogènes de l’élargissement spectral et de caractériser les processus de déphasage, de relaxation des populations et de diffusion spectrale. Parmi les résultats obtenus, nous avons mis en évidence l'influence des phonons spécifiques aux matrices moléculaires (ex : libration de N₂ et rotation de CH₄ ) sur le déphasage vibrationnel ainsi que l’influence de la transition de phase du méthane solide à 20 K sur la dynamique vibrationnelle. Nous avons également montré que la dynamique vibrationnelle était dépendante du site cristallographique dans lequel est piégée la molécule. Enfin, en excitant plusieurs modes de vibration simultanément, nous avons pu examiner les couplages intramoléculaires. / We built an experimental set-up in order to generate infrared stimulated photon echoes at the femtosecond timescale. The purpose is to examine the short time vibrational dynamics of metal carbonyls (W(CO)₆ and Fe(CO)₅) trapped in cryogenic matrices (4-50 K). This environment, resulting from the condensation of a gas mixture containing the impurity and an inert gas (N₂, CH₄, Ar, etc.), is well suited to study systems in their ground state. An excited molecular vibration is always damped in the time domain. It corresponds in the frequency domain to a broadening of the absorption line. The study of the vibrational dynamics aims at examining the physical causes of this spectral broadening. Typically, there are three kinds of causes: intramolecular phenomena, interactions between trapped molecules and interactions between the impurity and the environment. Photon echoes allow distinguishing between the homogeneous and the inhomogeneous contributions of the spectral broadening and characterizing dephasing process, population relaxation and spectral diffusion. Among the obtained results, we highlighted the influence of phonons that are specific to molecular matrices (ex: N₂ libration and CH₄ rotation) on the vibrational dephasing. Moreover, we observed the influence of the phase transition of solid methane at 20 K on the vibrational dynamics. We also showed that the vibrational dynamics depends on the site in which the molecule is trapped. Finally, when exciting several vibrational modes, we are also able to study intramolecular couplings.