How to generate 2-characters long words effectively?

List<string> GetWords(IEnumberable<char> characters) {
    char[] chars = characters.Distinct().ToArray();
    List<string> words = new List<string>(chars.Length*chars.Length);
    foreach (char i in chars)
       foreach (char j in chars)
          words.Add(i.ToString() + j.ToString());
    return words;
}

IDictionary<string, string> dictionary = GetRealTwoLetterWordDictionary();
char[] availableChars = new char[] { 'a', 's', 't' };
string[] combinations = GetAllCombinations(availableChars);
IList<string> results = new List<string>();

foreach (string combination in combinations)
{
    if (dictionary.ContainsKey(combination)))
    {
    	results.Add(combination);
    }

    string reversed = combination.Reverse();

    if (dictionary.ContainsKey(reversed)))
    {
    	results.Add(reversed);
    }
}